2003 pontiac grand prix still overheating

My car started running hot about three weeks ago. Heater would work on and off. Checked water and had oil in it. Replaced intake and head gaskets. Flushed Radiator. Ran fine for day or two. Started running hot again. Heat did not work at all. Changed thermostat. It was stuck. Flushed again. no change, still running hot. Replaced water pump. No change. No oil in water, heat still not working even after new thermostat. Can idle for a while but when accelerated or tr to drive it instantly begins to run hot. Not sure what else to do. Any advice would be greatly appreciated. Thanks