1993 Pontiac Firebird My car overheats irregularly

I have change my thermostat twice. My first overheating event, I replaced the thermostat with an off the wall one at pepboys and it kept overheating, I turned it into pepboys (had no choice) and the system was fine after I picked it up from the shop until I turned my vehicle off and then on again and it overheated once again. I checked many possibilities, my fans are bypassed with a custom switch where they are always running, I wanted to put a better thermostat than the pepboys one so I purchased it and I went to drain the coolant from the radiator and it was coming out brown, there was no oil in it, just brown and a little dirty. So I took it to the nearest auto shop which is a shop on base, very respectable shop, to have my radiator power flushed. They figured out that my thermostat was bad so they replaced it with the one that I was wanting to put in myself. They then ran a diagnostic that they came up with that my head gasket is blown (the radiator was not power flushed) I am not loosing any coolant anywhere, and I dont want to believe them. The overheating isnt consistant, it will be just random that it will want to overheat and then it doesnt. What should I do?