Today i was setting the timing on my 350 and when i put my hand on top of the distributor to set the timing the timing mark wound jump all over . I had to push on the vacum advance canister to get it set . With my hand off everthing is stable but as soon as i touch the coil the mark blinks off and on and is not stable . Also when i do this nothing happens to the motor rpm and the car runs ok. What might cause this to happen? Thanks .