2003 Chrysler Concorde, 145M miles. Car overheats and cools down sporatically while driving. Will run normal at start up and them spike to overheat. Stop, turn off engine, restart in five minutes and returns to normal temp and then overheats again. Changed thermostat but still repeats the problem. Belts, hoses, and coolant levels & pressure are OK. How to fix?

You say the head gaskets are fine, yet the compression is not.
How do you know the gaskets are not blown ?
Normally, when an engine runs hot, the gaskets expand and cannot contract, and the heads warp so they are not flat. You loose compression and usually have coolant in the cylinders.
You can also loose compression but not have coolant leaking into the block. It depends on what warped.

check your fan relays and temp sending units...turning on the heat will help dissipate heat from coolant. turning on a/c will engage secondary cooling fan...check your relays and sending units

Sounds like an electric cooling fan issue. Make sure they are working when you turn on the a/c. If they are not, you have a fan motor, relay, or fuse problem. Hope this helps, TJ

Well if its getting hot and boils change the thermostat.once it overheats the thermostat is different and its character is altered. is there coolant leaks that will need to be addressed. look at the radiator cap and inspect it are the seal worn Youll need to replace it if it is , now how does the coolant look like is it rusted in there.that will need to be flushed. now if you are driving it does the cooling fan come on.do you want to check to see if it comes on with the car running turn the a/c on does the fan come on if it doesnt then you need to find out what happened to it. here is what causes a car to over heat : coolant leaks,radiator plugged,thermostat,non operating cooling fan, radiator cap that cannot seal the system properly .

At idling speed an engine does build up a lot of heat and the cooling fan will kick in. In slow moving traffic or traffic jams the temperature gauge can touch the red - particularly on hot days. The reason it cools down when you start moving is because of the air flow through the radiator.

Presumably there are no leaks from the cooling system otherwise you would have mentioned it. In normal circumstances the fan will not be running as you are driving at speed, as the air-flow through the radiator is sufficient to cool things. The fan only kicks in to get rid of excess heat - and this usually occurs at idling speed or after you have parked the car.

If the fan is running all the time as you drive, this points to either a fault in the fan switch, or the car is running too hot. presumably in normal driving the fan isn't running and the temperature gauge reads normal?

It is common - in stationary traffic many cars overheat (particularly big engined models) try to stall and 'cut out'. Restarting can be difficult until the engine cools down.

Is your car overheating in normal driving conditions or just at idle speed? Overheating in normal driving conditions can be caused by things like a failing water pump, blocked radiator, collapsed hose, faulty thermostat or, in the worst case scenario, cylinder head problems.

Overheating at idling speed is 'common'. Check your coolant level. If your car isn't using/losing coolant then there probably is no major problem. You can flush out the cooling system and refill with new coolant - and also check your radiator. Are the cooling fins crumbling with age? Or maybe they're partly clogged with insects and debris from the road? A blast with a hosepipe wil sort that out ..

The question is how much does your car overheat in normal driving? If it doesn't .. it appears as though you have nothing to worry about as such. Most cars have 2 speed fans... the 2nd faster stage kicks in at some point dependant on engine temperature. Perfectly normal.

there are no reset codes, did you bleed the air out of the cooling system? if not do this procedure, if that doeasn't fix it u have a failed air temperasture control blend door motor actuator.
The generic method of bleeding air from the cooling system is to use a floor jack and raise the front of the car as high as poss, then fill the coolant recovery tank full and run the engine until it just starts to overheat, high end of normal zone on gauge, then shut the engine down and run cold water over the radiator core, this will self bleed the system and the coolant will be pulled from the recovery tank, repeat as necessary, never let the recovery tank run dry or more air will be pulled into the cooling systeml ,

u have a leaking head gasket, the fan is on because there is air pockets in the cooling system caused by combustion gases leaking in, the rough run is coolant getting into the cylinders.

with 17000 miles thats not normal

Car overheats then engine dies going down hill coasting. When try to start, starter spins as if no gas or fire to the plugs. Can this be a sensor flipped by too much heat or lack of coolant ? If so, how do you reset the sensor ?

did you bleed the cooling system? check the fans?