Chrysler Repair: 96 2.0L DOHC sporadic overheating, chrysler sebring, mechanic friend

Question
About a month and a half ago, the timing belt snapped on my 96 Chrysler Sebring LX (2.0L, manual transmission).  A mechanic/friend took the car apart and replaced the timing belt, oil pump, water pump, all gaskets and piston rings.  He glass beaded the pistons to make sure they weren't broken, and took the head to a machine shop.  He also cleaned pretty much everything.  The day I got my car back, it began over heating.  I took it on about an hour long drive.  The thermostat would almost reach the "H", then it would go back down.  This only happened when the car was actually driving.  The car does not get hot at idle.  My friend thought it might be the thermostat, so he changed that out.  The next day, the car didn't have any problems on my 10 mile drive to work, but it heated up on the way home. (In about 35 degree weather, heater was on, and blowing hot air).  I was watching the thermostat as it began to rise, so I pulled over, and heard something that sounded like water being poured into water (pretty loudly).  The thermostat began to go back to normal as I turned the car off.  I popped the hood, and didn't see any steam, or leaks.  I checked all of the hoses and none of them were wet.  My mechanic has NO idea what this could be, since everything is pretty much new and clean.  Do you have any suggestions?

Answer
Hi Katie,
It could be several reasons for what you have observed. The simplest possibility is that there is/was air trapped in the cooling system.
I would begin by checking the level of coolant in the radiator and if it is low add to fill it to the cap opening. Then put on the cap and check the level in the overflow bottle and if necessary fill it with coolant to the 'cold' mark. Then see if it is still doing the wide oscillation in the gauge and/or the water bubbling.
Also notice if the electric fan on the radiator is coming on when the gauge is reading warm to hot, which it should. If not then that might explain the problem.
If it is, then there are several possibilities. The radiator itself may have partial blockage of its tubes which might be remediated by flushing it out. Also inspect it from the outside through the grille to make sure it isn't blocked by leaves/debris as far as air passage is concerned.
Finally, notice when you start from cold whether you see alot more white smoke coming out the tail pipe, and which persists for longer than you remember ever seeing in the past. Let some of the smoke condense on your hand and then smell it to see if it has an odor of anti-freeze or is pretty much odorless like water. If it smells of anti-freeze then there is an internal engine leak that the person who did the re-build will need to consider as to how to correct that.
It would also be noticed that the coolant is disappearing without steaming out the overflow bottle or dripping on the ground under the engine, were this the cause. Also check the oil to see that it is not cloudy or milky which is also a sign of an internal leak in the engine.
Those are the signs/symptoms/techniques to investigate this issue.
Sorry for the delay as I just noticed your question in the "pool" to which it had been referred.
Roland