95' overheating mystery-please help

[mollbells]

My 95' Grand Am SE 3.1 L V6 with 130,000 mi gradually overheats to about 240 when the car is started cold. During this time the heater blows cold air, after about 15 seconds of being the the "red zone" the temp gauge drops very quickly to normal operating temperature, and heat pours out of the vents.
The temperature gauge will heat to around 220 and then drop quickly to normal several more times during vehicle operation, the heat will continue to work. The temperature remains stable after this even on 250 mile long driving trips. The heat doesn't seem to be as hot as it used to be, but not completely sure. The coolant system seems to be leaking slowly (1/2 gal in 2 months) but there are no visible leaks. We saw bubbles rising in the coolant resivoir by shining a flashlight on it at night, while the car was idling. No water in the oil. No definate sign of coolant coming out of exhaust, although not completely convinced as it is winter.
The thermostat was replaced twice in last six months. The coolant system was pressure tested at GM dealership and was fine. They also said the cooling fan was operating normally. Intake manifold gaskets replaced 2 yrs ago (as preventative for a slow leak).
I have been told that the head gasket needs to be replaced, but this is a big job and I want to rule out less intense problems before ripping the car apart. This problem has been going on for about 4 months now. Please help if you have heard of this problem and know the solution.

[GTP Dad]

Welcome to AF! You don't say what engine or how many miles but this sounds like a water pump issue. It appears that the water is not circulating properly until you start moving and get the engine rpm's up a little. Even though there are no leaks externally that you can see it is possible that the pump is slowly leaking and causing the issue. Also since you indicated that the thermostat has been replaced twice in 6 months I don't think that is the issue. Spend a little money and replace the water pump and I believe the problem will go away.