Have a strange problem on my '85 Bronco 351W. It runs great after the complete rebuild except for one problem. With the lights on I get a stumble or misfire at low RPM. If the radio is turned up (400W AMP) it's worse. The other night the lights were on, radio on, defrost blower on high and my son put up his electric window... I was going about 35MPH, 2000 RPM +/- and the truck stalled. It started back up right away and the battery stays charged. Any ideas?