1995 Ford Mustang No water flow!

Ok where do I start.I recently rebuilt my engine and started it up, I know have a overheating problem that I cannot solve. I have filled up my radiator and had the car running it takes about 15 20 minutes to overheat I have swapped out thermostats about three times thinking that the thermostats were stuck and wouldnt allow coolant through, and I have a brand new water pump. I took the thermostat out of the car and the coolant flowed throught the radiator correctly after burping all the air out of the system but then driving down the road after about 25-30 mins its starts getting pretty hot, I think this is because I have a stock radiator on a really beefed up engine but then I put the thermostat back into the engine to see if the air being in the system was the problem but when I put the thermostat back into the car the coolant just sits there then rises up and bubbles air out of the system then sucks coolant down into the radiator, ill fill the radiator up again and itll bubble up then suck it down over and over about 20 times, but the car is heating up when this process is occuring. The engine gets to the point where I have to turn it off for fear of damage but the coolant never flows like it suppose to inside the radiator. It makes me think the thermostat is never opening, one individule suggested that I may have put the head gasket on backwards but I rele dont think so because why would the coolant have flowed with the thermostat out of the car I would think that the coolant wouldnt flow and the car would just heat up if that is the head gasket is the problem. I have been using the stock gauge to determine if the engine is hot. Please gimme something to work with im out of ideas and im getting really frustrated. Thanks.