Here's the problem,when the car is cold it runs at 750 rpms. when it is warm it runs at 500 to 550 rpms. It has a fresh tune-up. The IAC was replaced with no difference. There are no codes stored. It runs great going down the highway. When I place the shifter in reverse [when warm] the motor rpm bogs then comes back to 500 to 550 rpms, the same happens in drive. At a light it doesn't bog it just stays at a low idle. Any Ideas would be greatly appreciated.