1995 Chevy Corsica Overheat then not

After changing the water pump and refilling the system with fresh coolant. When first started after being allowed to cool the car will warm up and the temperature will continue to climb all the way up to the H indicator on the gauge. It will sit there for about 1 minute and then it will drop back down to less than the first quarter mark on the guage.
This is where it has been running since I replaced the thermostat with a 180 degree model at the same time as I replaced the radiator in the fall of 2008.
Also the low coolant light has been on and stays on since I changed the water pump, despite the resevoir being full. Coolant is being pushed out from under the new resevoir cap when it heats up, but probably because I have overfilled it. I also installed a new heater core about 2 months ago. I have the heater controls turned to full heat, and as the car warm up it starts to produce warm air, but when the temp starts to climb higher the heater temp cools off. This seems to me to mean that I am not getting the cooling system filled properly, but I have removed the bleed valve while filling and also unhooked a hose higher on the block than the bleeder valve and was able to get coolent to run from both places. The low coolant light came on and stayed on for a week after I replaced the heater core, but the temp guage acted normally at start up and all other times. The temp only rises to the "H" once at start up (after running around 5 minutes)and then runs normally no matter traffic or speed till the car is turned off and cools down completely.
Thanks for any help with this question. Its a weird one.