Try disconnecting vacuum line to distributor, breaker plate may be worn and opening or closing points when activated. there also may be break in electric or earth lead to points that make or break when breaker plate moves. other souces are exhaust leak close to manifold bringing in fresh air and igniting unburnt fuel in pipe. Cracked dist cap, plug leads breaking down under accel then igniting unburnt fuel on decel. A quick hookup to tunescope will instantly show up fault.