Let's ask a few questions about the engine setup. 1. Does it have an aftermarket cam? 2. are you running vacuum advance and is it connected to manifold vacuum? The reason I ask if you already have low manifold vacuum, and you add a small load, the vacuum drops and the ignition retards (due to lower vacuum) engine slows down more. Then the vacuum drops and the ignition retards (due to lower vacuum) engine slows down more. Until it stops 400 rpm later. Need a vacuum and voltage reading while the event is taking place to really understand what is happening.