Betting you have very light springs in the dist for a fast curve? The newer style MP dist had light springs and they start to advance at around 800 rpm so at idle when your try to set timing the weights are bouncing due to light springs. If you use an advance light and set total timing at 2500 rpm Ill bet timing mark does not move or bounce, if this is the case you need a heaver spring or even both in your dist. Ive got a sun dist machine and see this all the time and a spring cures it.