1998 Subaru Outback Temperature Gauge Fluctuation

I recently bought a 1998 Subaru Legacy Outback with the 2.5L engine. The car recently overheated and when I took it back to the dealer I bought it from they replaced both head gaskets, water pump, t-stat, timing belt, crank seal, and cam seals. During this process the cooling system was flushed. When I picked up the car I noticed that the temperature gauge fluctuates between the normal operating temperature (i.E. Slightly below half on the gauge) and 2/3 of the way to the H on the gauge. The dealer told me that it was probably air trapped in the system and that it would slowly bleed and purge itself out. After a couple of days I took it to another garage and a mechanic showed me how to bleed the system with the bleeding valve located on the passengers side of the radiator. I have been bleeding the system for the past couple days and the problem seems to be getting better but the temperature gauge still rises a bit when the car is in drive and stopped. I am concerned because it is winter here in NY and the gauge never flucutated at all before this problem. I am worried that I will have a more serious problem when it is summer here. My technique for bleeding the system is to open up the bleeder and the radiator cap when the system is cold. I then fill up the coolant through both of these holes. I start the engine and wait for it to warm up. I subsequently continue to fill the coolant once the t-stat opens up. I have done this three to four times and everytime I open up both of these holes the coolant in the radiator is down. Shouldn't it be full from vacuuming in from the coolant reservoir as the engine cools down? Any help with this problem would be appreciated. Feel free to ask any questions you may have as I am pretty familiar with cars.