Eratic idling and misfiring under pressure

A lean system suggests it is possibly not getting enough fuel. check electrical signal to the injectors on both sides of the engine and compare the two sets. Does the engine run and idle?
Here's a simple way of checking the injectors without removing them.
*NOTE* You will be working with the engine Running so BE CAREFUL of moving parts and the engine getting hot.
You will need an extra long screwdriver or a long metal rod to do this. With the engine idling rest one end of the screwdriver on injector number One. Put your ear on the other end. Note, This will be louder than you expect. A working injector will be a very fast and even ticking noise that gets faster as you rev the engine. Repeat this on injectors 2 and 3.
A rough or irregular tick suggests a dirty or faulty injector. If all three are rough check you fuel rail pressure. Have you tried a dose of injector cleaner through the system. With the engine cold add the correct full tank dose to a quarter tank of fuel and let it idle up to working temperature, (about 10 minutes). DO NOT rev the engine as this thins the mixture making it less effective. Take the vehicle out for a good drive on slow roads and fast roads but don't push it hard as that also thins the mixture. Fill the tank, put a second dose of cleaner in and drive it as normal until the fuel light comes on. Refill the tank and drive normally.
It could also be worth you checking the electrical and mechanical timing on the engine. Even if it is slightly out it can cause issues.
Check the engine filters as well. A dirty Air/Fuel filter can have a bigger effect than you would expect

It isn't wise to get too preoccupied with fuel pressures - ok the pump needs to maintain the right pressure but it is also important the pump delivers sufficient volume of fuel that the pressure is maintained under full throttle conditions with enough reserve circulating back to the tank to keep the system cool and prevent vapour locks.
A healthy pump will fill a half gallon jug in a few seconds and that test is the essential other half of the fuel system test.
A reduced volume or pressure could indicate a dirty fuel tank...

The pressure test isn't complete until the peak pressure has also been tested and the pressure regulator vac on and vac off proved ok. The rail pressure should be maintained with the pump off and the supply closed - if it doesn't, it indicates a leaking regulator or a dribbling injector.

Once the fuel delivery has been passed ok, the fuel injectors should be removed and tested. An injector has a useful life on average of 70/80,000 miles, after which the spray patterns deteriorate - a poor spray pattern has larger droplets that burn much slower and behave like a lean mixture.

many people mistakingly believe that misfires are the results of ignition problems only. There are several other variables that have to be considered . They are fuel pressure and injector performance. If the fuel pressure is too low then the injectors squirt instead of atomising an the resultant air/ fuel mixture will not combust properly resulting in misfiring. Plugs used in the engines manufactured in the last 12 years will generally out last the engine . Quality HT leads if separated correctly will last many years . But injectors which are subject to electronic problems and dirty fuel (gum and tars not removed by filters) will be the culprit in most engine misfires. So have a pressure test done on the fuel manifold to check the fuel pressure regulator, pump and filter. have the injectors cleaned and regularly maintained .

misfires are more from problem injectors, low fuel pressure or injector wiring than from plugs
have a fuel pressure test done to check for problem pressure regulator , problem fuel pump , blocked m/dirty filter
one other area of concern is the EVAP system where the purge valve doesn't vent the tank properly and causes low fuel flow rates

Could be a number of issues. Fuel pressure or regulator issue, but...
If you have a coolant sensor problem, as in reading wrong, and it tells your computer it is colder than it actually is,,,you will get more fuel delivery.

First, take a quick sniff of the exhaust when the engine is idling. If you smell gas there, check your spark plugs and wires to eliminate misfires, then check for leaky fuel injectors (leaking excess fuel into a cylinder).

If the exhaust smells normal, you'll need to inspect the fuel delivery system. The most probable place for a leak would be the fuel lines under the car; it's old enough that a line could be rusted through and have a pinhole leak somewhere. If you suspect a spot but want to verify it by turning on the engine to pressurize the line, be sure to wear safety goggles. You don't want gasoline sprayed in your eyes.

po171 is lean on bank 1--===causes --MAF/VAF sensors --injectors ---HO2S (heated oxygen sensor)
po300 is random misfires and po301, 2, 3 4 5 6 etc is the cylinder that is misfiring
most likely that the fuel pressure is below the required pressure either from the pump or blocked filter or the tank not venting properly--- (EVAP) problem
check the fuel pressure ( minimum of 45-65psi) --problem pressure regulator , pump or filter
replace the heated oxygen sensor on bank 1 ( in the exhaust manifold before the cat converter) as it reports to the ECM to get the correct air/fuel mix

this is one of those situations that a professional diag would of saved you alot of money. Its possible you just got bad gas and your injectors are partially clogged. An 02 sensor would not give multible misfires. EGR would only misfire at idle if stuck partially open since its suppose to be open cruising. Fuel filter would give you a starvation issue but if the misfires happens at idle then the filter most likley is ok.
Data from a professional data scanner will store the conditions when the misfire occurs. Also for a chevy each cylinder will have a misfire counter and one can see the misfire happening as it happens. If its idling and misfiring and gets better as you rev it up most likely you have clogged injectors or a intake vacuum leak. You can test for a intake leak by spraying the intake area with water(idle will faulter if it draws water in), or use brake cleaner (idle will raise when fluid gets drawn in).

If no leaks are found I would do an injection service or at least add two bottles of techron to a full tank, drve till half a tank then top off again.

Also if the catalytic converter is clogged the backpressure could cause all sorts of misfire issues. Removing the two upfront 02 sensors and driving will indicate the exhaust is clogged or not. We put a pressure sensor in place of the 02 sensor. If the pressure is above 5psi this would indicate some blockage. It should be below 3psi at 2000 rpm. Also if you have K/N filter the oil from it will coat the maf sensor wires in time,, (always). You can remove the sensor and spray the wires with brake cleaner to clean. DO NOT touch them. They can be brittle. Look at the wires for color. They should be shiny silver. If its coated the PCM will not see the air volume or accurately see the air temperature. Without this information the PCM will not add enough fuel which will create a misfire.

Check the ohm's reading on the fuel injectors, they should each read around 14-16 ohm's cold for most 1997 GM engines, and if any fuel injectors are not within range replace them.

Possible fuel delivery problem, fuel pump, fuel line, clogged fuel filter, leaking injectors or injector O-rings, faulty pressure regulator etc. Check your fuel pressure.