Ford Mustang: 91 mustang high idle, clutch linkage, code scanner

Question
I have been battling a high idle issue since I swapped out engines, like for like 5.0s 5 spd. Once car is running and in neutral let clutch out and idle goes to 2500. push clutch back in idle comes down to normal.  If I put a washer in the safety switch for the clutch, idle stays where it should, however when in gear and I start to let out the clutch the engine will rev slightly. But once rolling and the clutch is disengaged the idle will be normal. I have changed temp sensor and tried to get auto shop to get codes but there code scanner just resets when the engine is started.  Please HELP!!!

Answer
Check your clutch linkage and throttle cable. Something maybe interfering with those or maybe with each other. Have someone under the hood watching when you have it running and let the clutch in and out. Also watch for excess movement of the motor. Check all motormounts, make sure they are not loose or broken.