1995 Chevy Camaro my heater and coolant.

I hydro locked the engine to my car in the summer. I put a new engine in it and it runs fine. When I beat a little bit on my car coolant pressure builds up and sometimes sprays out of the resivoir. Ill have heat until I beat on it a bit and then it goes to cold and my temperature gauge goes up and down. After I release the pressure from my radiator the heat will work untill I beat on it again. People have told me its my head gasket. I flushed my system with degunking liquid from napa. I also put a new thermostat in it. When it sprays coolant, the coolant is dirty and rusty looking.