Why would your car shake when you are idling and have the AC on but not as much when you are driving?

The car vill vibrate because the A/C system uses a compressor that runs off the engine's pulley system. The compressor is quite hard to turn and as such will actually use a few extra horsepower to run it, this results in a drop of the rpm from idle. Depending on your car, most petrol engines idle at approximately 800 - 900 rpm. When the A/C is activated, the compressor's cluth will engage and put extra load on the engine, thereby lowering the rpm. When an engine's rpm drops below about 700 - 800 rpm, the car will vibrate because of the balance of the pistons and crankshaft in the bottom end of the engine thus creating a vibrating effect. Engine's are balance using counterweights on the crankshaft and are set at a good balance with engine rpm being about 800. I hope this has helped you. rest assured, there is nothing wrong with your car and you will never resolve it as long as you have the A/C turned on.