Combustion in coolant

From what you have described you have radiator coolant leaking into the engine cylinders.

This will not be rectified by changing the gaskets on the rocker covers. That just stops engine oil leaking out from around the edges of the rocker covers.

The coolant will be leaking internally due to a failed cylinder head gasket and/or a crack in the cylinder head(s) and/or even a crack in the engine block. You will most likely also have combustion gasses leaking into the cooling system passages from one or more cylinders. I expect there will also be some coolant finding its way into the engine oil.

If you have allowed the radiator coolant to run low and hence for the engine to overheat at any stage that will cause the problems you have now.

You need to have tests run to check if you have a failed head gasket(s) and in that event the cylinder heads will need to be removed, the head gaskets replaced and, while removed, the heads checked for cracks, checked for any warping and overall condition. The engine block will also need to be checked.
If you run a compression test on all cylinders you will likely see the compression lower than spec in a number of them. A leak down test on each cylinder will show which cylinders have coolant leaking into them(and combustion gasses leaking out into the cooling system) and certainly confirm the need to remove the cylinder heads.

If you start the engine cold with the radiator cap removed, once the coolant has warmed and begins flowing from the engine via the top radiator hose to the radiator you will see bubbles in the coolant if there are combustion gasses leaking into the cooling system as described. This is an indication, (because the bubbles can also be air not properly bled out of the cooling system) but you still need to run a leak down test.

You need to have the problem checked out immediately as you do not want to be running the engine in this condition. Make sure you also check the level of the coolant in the cooling system and top it up when the engine is cool if not full. Coolant can escape quickly under normal cooling system pressure when the engine is running.

White smoke usually is coolant burning. Is engine loosing coolant? Small pop could have been head cracking, or possibly head gasket failing. If you are loosing coolant, time to remove the head to see what failed.

might be cylinder head gasket leaking to combustion chamber,small leak that it can sip into combustion chamber and burn but the coolant will not burn completely so will stick to spark plug and cause miss fire.

Buy a chiltons manual that's specific the the cyl heads your trying to remove. I'm guessing your talking about heads on a car ? By not including a make, model, year , engine size , diesel or gas, overhead cam, naturally aspirated, turbo, fuel injected, carbureted there is no way to answer the question you are asking. Your biggest concern should be on how to reinstall them. Reinstalling heads are far more complex than removing them. All heads are different in the pattern in which they are tightened down ( to prevent warping ) . A specific amount of torque must be applied to each bolt. Some bolts require thread sealant because they pass through the water/coolant passages in the engine. Failure to do that will result in water leaking into you combustion chamber thus ruining your engine as well as your new head. Also the thickness of the head gasket must be correct because to thin or to thick of a gasket will change your engines compression ratio . The gasket thickness can also change your cc's in the combustion chamber. A gasket that's to thin can also allow the exhaust or intake valve to hit the top of the piston on the compression stroke. If your dealing with an overhead cam or dual overhead cam engine then the proper setting of the camshafts when reinstalling the head or heads is crucial . The above only pertained to gas engines. If you have a Diesel engine then things change dramatically . The pressures in the cylinder head combustion chamber area on a Diesel engine is unbelievable . One small mistake or oversight on reinstalling a head on a Diesel engine can be catastrophic .

a cylinder compression check is needed. You can do it yourself with the proper compression gauge available at auto parts stores. This check will tell you if the cylinder head gasket is leaking. Another good test is the Lisle 75500 combustion gas leak tester. This is less expensive but will show if combustion gases are leaking into the engine coolant--a sign of head gasket failure or a cracked cylinder head.

Replacing a cylinder head gasket is a miss or a hit. In some cases, just replacing the gasket may work. But if the cylinder head is bend, dealer may ask to replace it, because "machined aluminum heads rarely works". In this case the cost may be more than $1000. It is a good idea to send the cylinder head for inspection, in a machine shop. They charge $150 for inspecting, checking, cleaning and if necessary machining the head.
If the timing belt will be replaced this is an extra cost. If someone will just replace the gasket, on a bend cylinder head, it is more likely to blow out again. This problem can be solved if the gasket be replaced properly, checking and taking care for the cylinder head too.
As the coolant warms up, it expands and for that reason you have two marks at coolant bottle. Down cold and up hot. It is normal for the coolant to overflow from coolant bottle, if it is filled to the upper mark with engine cold, way up the cold coolant mark. Let engine to cool down overnight and set the level at coolant bottle at the lover mark cold. If it will overflow again this is a bad sign. The pressure at combustion chamber varies from 75 in idle up to 150 or more in higher RPM. If the cylinder head gasket is blown, then gases are blown into the cooling system, increasing the pressure, the radiator cap pressure relief valve is open and coolant is discharged in the coolant bottle. In such cases it is common for the coolant bottle to boil. You can observe gases discharging in the bottle, mixed with coolant, from the cooling system. These are symptoms of a blown cylinder head gasket. Because you replaced it once, there might be a bad (bend, deformed) CYLINDER HEAD too.
To find it out you have to perform cylinder compression test on your car (PROFESSIONALLY).
1. Start with a normal ("static") compression test. To eliminate rings, valves, holes in pistons, that sort of things. A normal cylinder balance test is also helpful (so you know which, if any, cylinder is presenting a problem). Engine should be warm.
2. Put all spark plugs but one back in. Ground that plug wire to prevent module damage. Disconnect that injector on a port fuel system.
3. Put your compression tester into the empty hole. The test can be done without a Shrader valve, but most people recommended leaving the valve in the gauge and "burping" the gauge every 5-6 "puffs".
4. Start the engine and take a reading. Write it down
5. Now goose the throttle for a "snap acceleration" reading. Reading should rise. Write it down NOTE: Don't use the gas pedal for this snap acceleration. The idea is to manually open then close throttle as fast as possible while without speeding up the engine. This forces the engine to take a "gulp" of air.
6. Now write down your readings for at least the bad cylinder (if there is a single bad cylinder) and maybe 2-3 good ones. Make a chart like this: CYL STATIC COMPR IDLE -RUNNING COMPR - SNAP Cyl 1 150 75 125 Cyl 2 175 80 130 Cyl 3 160 75 120 Cyl 4 160 80 125
7. ANALYSIS: Running compression at idle should be 50-75 psi (about half cranking compression). Snap throttle compression should be about 80% of cranking compression.
Consult a professional with the readings. If the gasket is to bad, you may have very low compression at lease in one cylinder.
If you will get a positive confirmation for a bad cylinder head then you have two options:
REBUILD THE CYLINDER HEAD or REPLACE IT.

sounds like a blown head gasket. coolant being pushed into combustion chamber and combustion being pushed into the cooling system. combustion gasses pressurizing the coolant system will cause overheating.

To pin point a blowing headgasket you will need
1)coolant system compression tester
or
2)chemical reaction test for combustion test gas
If it's the org. engine from 1994,with over 150,000 then blown headgasket is a good possiblity.
Tow it to the shop and let the pro to do the coolant pressure test and combustion gas test.
A full head gasket job with head re-surface (if it can be re-use) is worth it.
Shop around on well estiblished independent shops.

it could be a valve or the compression rings on the piston may need to be changed

Did you have the head machined/shaved before you replaced the head gasket with a new one? If not--do over---machine head---replace gasket.

BUT, before you even bother, have you considered the fact that the BLOCK may be cracked? You can check this by looking at each spark (plug for combustion residue of coolant or oil) of each cylinder separately.