inconsistent overheating...puzzling problem

My 97 neon has been overheating for about a month. The first time I took it in to the mechanic, my coolant was boiling and leaking out and they replaced the thermostat and everything seemed fine until I tried to turn on the AC or the fan. Once I turned on the AC or fan, the temperature would immediately spike. So I took it in to the mechanic again where they replaced the head gasket, water pump, timing belt, radiator cap, and sensors. It worked fine for a week with the AC on, but now it is starting to overheat yet again. The overheating is not consistent...sometimes the temp would spike and gradually lower and sometimes I have to crank the heat to vent it from the engine so that I can get home. I've been watching the coolant levels and it's not leaking anything. The engine fans are running fine. Now it sometimes begins to overheat without the AC on. If anyone can give me some advice on what the problem is that would be great!! Thanks!!

are you SURE the fans are on when its overheating? it sounds as tho the fans are cutting on/off..

if its not that, mabe its crap in the thermostat plugging it up? have you gotten a system flush? that might help..

Thanks for your reply!! Yes everything was tested and flushed by two separate mechanics. All of the sensors, relays, fans, pressure...everything comes out ok in the diagnostic. The second mechanic drove the car around and he suspects it's a cracked cylinder because the temp gauge fluctuated when he drove it around, but the first mechanic swore he did pressure tests on the cylinder head before he put a new head gasket on and that it came out fine (he had to machine the cylinder head because it was warped from an accident). The first mechanic drove the car around as well and couldn't find any problems with overheating. I think my main problem is that it is inconsistent so everyone is having a tough time diagnosing the problem. One day the car will drive fine...the next day it's overheating. It seems to run better on colder days though and the first mechanic was only driving it around in colder weather. It seems to also make a difference how much start stop there is during my drive.

wow. that sucks... i have a few questions tho,

is the coolant level getting any lower? is there foam in the oil? if there is, the first mechanic didnt do the cylinder head machining right.. or you have a cracked block.. but if there isnt any oil floating in the coolant/ loss of coolant there shouldnt be any problems? (am i missing something here?) cuz if the coolant level is OK, the fans turn on, the thermos lets the water through, and its not leaking coolant anywhere..as far as the cooling system is concerned, its working fine.

have you ever checked to see if a heater/coolant hose was being pinched as the fluid got warmer? im guessing the hoses all got changed when you did the flush, etc..? how about the radiator fins? are they all non-clogged? the radiator IS hot when you touch it tho, right? as in it is getting heated.. and letting it out into the atmosphere..

thats a weird problem..

btw, you should get a external IR thermometer or something..and make sure your temp gauge is ok

I was having a similar problem with my 99 neon. My mechanic kept checking and couldn't find anything wrong, until he pulled the fans and shroud to get a look at the radiator. The interior side of my radiator fins were extremely corroded and clogged up, so very little airflow crossed the radiator. Idling and driving for short stints was fine, but driving it hard as on an interstate or hilly area and it would overheat. Mine would cool down when I'd pull to the side of the road and let it idle though. Good luck with it.