I have an 06 pontiac Grand Prix, I have had the bypass elbows replace and a new thermostat aswell, idling my car overheats but when I apply the accelerator my temp gauge returns to it normal state.

There is air trapped in the cooling system. Modern GM cooling systems are tricky to fill. I believe there is a bleeder valve near the thermostat that may solve the problem. I've also hear filling the system with the car pointed uphill will help bring the air to the rad and release it although I've never actually tried it. One way or another you have to get the air out or you will continue to have problems.