when i start the car when the engine is cold with the A/C on, it drops about 400 rpms. If i then put the car in reverse, it will start shaking in the front end. If i turn the A/C off at that point, the shaking will stop and the RPMs will go back up. Could the broken engine mount cause this shaking? or is something wrong with my A/C. and whats up with the sudden power drop?