Overheating Help!!!

[2000GAGTNB]

ok I have a 2000 Pontiac Grand am GT 3400 SFI, the problem is I got the gaskets done in the car 2 and a half years ago now the low coolant light started coming on, when it first did we added coolant to it... but I noticed that very quickly I lose heat and the low coolant light comes on again. so I changed the thermostat and the water pump thinking they could cause the problem... well after changing those parts I filled the car with coolant again and started it up i opened the little valve on the engine to let air out and kept filling the tank... then I finally got some heat after filling this tank for quite a while. I took the car for a drive and abgout a 1/4 mile later no more heat and the temp gauge went 2 little lines over the 1/2 mark then again the low coolant light came on... I came back home thinking i mightve had more air in the system as when the light comes on I notices that the overflow tank spits out all the coolant in the system so i restarted the process and there is no pressure in the hoses but all of a sudden by filling over again I get heat and it seems to be running fine but when I put the cap back on and go for a drive I lose all coolant through the overflow hose and lose heat and the temp gauge goes 2-3 little lines over the 1/2 mark again now what coulr be this problem as I got a bit of heat again as soon as the gauge goes a little under the 1/2 mark I dont understand???

[xeroinfinity]

Welcome to AF.

Did you turn the heater on when you are purging the air from the system??

If not try that, open it up full blast.
Also might help if you elevate the front of the car slightly.
Keep the engine running with the resivoir cap off for several minutes.

The problem is that a water pump cant pump air very well.
So once its trapped in the heater core, its difficult sometimes to move it out.

You did put the thermostat in right, with the spring toward the engine?

Hope that helps!