2004 Chevrolet Cavalier Cavalier loosing coolant

2004 cavalier ecotec engine. Problem is that to me the coolant is not being circulated right.I'm loosing coolant out of the fill bottle while driving. Also I've noticed it builds a lot of pressure in the lines. When starting the car hose is soft but 5 mins into it it's rock hard and the only way to relieve pressure is to open cap slowly. If I leave the cap loose and give it gas the coolant starts to over flow. Changes water pump. Tstat. The radiator is clean and flows good. Engine temp sensor changed also.I'm running outta sloutions. Fan works like it should. Comes on during ac and when engine get over 200+.(Also as right now I got no thermostat in the car but it still gets to hot 200+. Can't figure it out. Other then that car runs great