2000 Mitsubishi Galant High idle

When I start car it idles at 2700 rpm when I drop it in drive it drops to 1300 rpms buy feels like it wants to take off still and it slept out of gear or doesn't shift correct. I have tore my engine apart looking for the problem from the throttle body to changing every gasket seal and vacuum line. I have a new throttle body comming with a iac and is already a just ed for my car hoping this will finally fix my problem after that u don't know what else it could be I checked the sensors for volts and the main ones that control idle are reading fine so I'm at a loss if this does not fix the problem.