Chevrolet Repair: Hot Gauges and No Heat (sporatically), coolant temperature sensor, 1995 chevy lumina

Question
I have been to 5 different mechanics and none of them have been able to fix the problem. My teenage son owns a 1995 Chevy Lumina with a 3100 6 cylinder engine. About a week ago, it overheated (not bad - he pulled right over when the hot and servicengine light came on!). We replaced the thermostat and the radiator cap and it worked overnight. Now the gauges keep going to HOT and the heater quits working during this time, too. After a minute the guages come down to the middle and the heat returns. We were told that there is AIR in the system and that this engine is very hard to get the air out of. We have several mechanics bleed it out through the two release valves. It works overnight and then right back to the gauges rising, the hot light coming on and the heater not working for 1 miute or so and then it all returns to normal - heat and all. We had it pressure tested yesterday for leaks and at 20 pounds it shows no signs of leaks. Again it worked fine for a while only ... HELP!

Already Tried:
1. 2 New Thermostats(first 195 degrees and now 180 degrees becasue the guages showed it was running hot) 2. New Radiator Cap 3. Pressure Testing the engine at 20 pounds to rule out leaks to the coolant system or gasket leaks. Also Also the COOLANT light will come on and off and the overfill canister is FULL. When the temperature guage shows normal we have heat, when it starts rising - it loses heat. It goes all the way up - the HOT light comes on and then it stays up for a minute or so with NO HEAT and then quickly falls back down to normal and the heat returns. We thought head gasket - but oil looks good and engine sounds and runs great otherwise. HELP!!!
We also put in a new Engine Coolant Temperature Sensor, too - because the fans did not seem to be kicking on soon enough. This could have been also been caused by the AIR in the sustem as well, becasue it was located at the top of the engine block. Are there any other SENSORS that may be bad or could the guages be faulty? I do not think that these things would cause the heater to go on and off though? Also some people think the water pump, but I have nevr heard of them working on and off with no leaks??

Answer
Hi Vicki.... these are hard to get the air out of... and it can drive you nuts... i found the best way is to remove a heater core hose and fill the heater core manually. use the line that is the higest...fill the core till it will not take any more fluid, reattach the line and make sure the rad and the overflow tank are full. start and run the car till it reaches temp and you have heat. shut off and allow to cool. once cool remove rad cap and see if rad is full... if not top it up.. run car again till you have heat and check overflow tank. if it's full then you should be ok now...drive car and see if it works now...if not try repeating process... air gets trapped in the heater core because it is lower than the engine.... also check that waterpump is working... i have see those where the impeller for the pump is broke off inside and although the pully is turning the pump is not pumping.... also the sensors do not turn fans on till engine gets over 200 some time as high as 205 ...hope this helps