Chrysler Repair: Neon Coling system/overheat problem, chrysler neon, head gasket

Question
Hi, I've got a 1999 first generation Chrysler Neon. Recently, the thermostat broke and we replaced it. But now we have a kind of strange problem. Sometimes the temp gauge climbs quickly and reports that the car is overheating. This occurs randomly and most of the time the temp is steady. Also, when we shut of the engine, the is a loud bubbling sound(sort of like when you use a water cooler) from around the header tank, followed a few slow bubbles every few seconds, as if there is a lot of pressure built up somewhere. We took it to a mechanic, but he cant find anything wrong. He also did a compression test and analyzed the exhaust gases as he thought it might be the head gasket, but compression was fine and there was no evidence of antifreeze in the exhaust (though there seems to be quite a lot of white steam to us). Also, he says it is losing water somewhere, but he cant figure out where. But we have noticed that the water level in the header tank keeps changing. Sometimes its a lot higher than before we started, and sometimes its a lot lower. So we're kind of confused. Some ideas on what this could be and how to fix it would be really helpful.

Thanks,
Rob

Answer
Either you have air in the system but I personally seen this before and do not think that this is the problem. I definitely think its a Cylinder Head or Gasket problem I see it all the time and personally I've seen those exhaust gasses testers work one time out of I don't know how many times I've seen them performed not very dependable. But those bubbles that you hear definitely is a sound that 9 out of 10 times relates back to a cylinder head sucking air into the engine somewhere sorry to tell you this but I would almost bet anything that its a head problem even without seeing the car