1991 Pontiac Sunbird heating/cooling/temperature system

1991 Pontiac Sunbird mileage: 111,000. Hello! I have an ongoing situation I hope you can help me and my mechanic with! The problem involves the heating/cooling/temperature system. I have had a lot of work done to it, including replacing the water pump, the thermostat multiple times, the radiator cap, flushes, etc. (The air temperature sensor and the serpentine belt were also recently replaced--not sure if this is related--and I think I am out of Freon--not sure if this is related either, but I don't want to leave anything important out.) In my latest round of repairs, my car was running very hot, and was leaking coolant, progressively worse, from a small occasional leak to the point where it was dumping out 3+ quarts a day. They found hole in a hose near the back and repaired it. Now, my car doesn't leak, but the temperature gauge is fluctuating wildly, all the way to the top and back down almost all the way. The heater is blowing, at best, cool air, and sometimes it is cold. This also happened the last time that it was fixed in December. At that time, I pulled over, let the car cool down, took off the radiator cap, and when I put it back on and kept driving, the temperature gauge did not fluctuate anymore, but the heater did not work at that time either. I assumed that the car had some kind of air bubble and just needed to "burp", for lack of a nicer term. It kind of got to the point where everything was okay, it was running a little warmer than I liked but the heater was working, but then the latest round of problems started. After the latest repair, I tried taking the cap off again and letting it "burp", but the problem isn't improving. My mechanic went ahead and put in a new thermostat. Could the problem involve the head gasket and/or damage to that part, or do you think it could be something else? I have been told by a couple of mechanics now that Sunbirds the age of mine had trouble with faulty gaskets or something to that effect. Is this the case? Also, do you have a very rough, ballpark idea about what repair might run? I have already had approximately 3-4 repairs of a different nature for this problem, and I am in such a bad position, because while I can't afford to keep putting money into this car, I can't afford to buy another one either. I appreciate your help very much!