I think most of the suggestions here are good ones. Not too sure I buy into the too low of an idle as my engines do not vibrate at a very low ( below 1400 ) RPMs. I see that you are a gas car guy. You wouldn't by chance be using your car fuel or plugswould you? Usually a gas car gets set up pretty lean on the idle mixture, this is done for quick accelleration. You may just be too lean on the bottom end and this could create pre-ignition which would cause vibration. Running car fuel with it's lower oil content can do the same and if that's not enough LOL. If you are using your car plugs,they could be too hot of a heat range and that can contribute to pre-ignition too.