temp/heat

i have a 94 gp 3.1....recently replaced the head gaskets and every since, it has been overheating...replaced the thermostat and when you start it, it always heats up really fast, sometimes it'll go all the way up to red (in the mean-time, the heater is not working) and stay there.....sometimes it'll go all the way to the red and all of a sudden drop back down REALLY quickly and all of a sudden, the heats working! any ideas? i just dont want to be driving it one day and expect the temp to drop and it doesnt and i ruin something else!

sounds like air trapped in the cooling system. These cars have a special coolant bleed proceedure that has to be followed to purge trapped air out. There are bleed screws on the t-stat housing and coolant transfer tube that are for this purpose.