I gave this some thought today. sounds like your coil is getting too much voltage. If it gets worse the longer it runs, it must be getting hot. If the lights being on cures it, then I would say either there is something wrong at the ballast or the voltage reg is causing the alternator to hike the volts too high. When the lights are on the voltage drops and the coil behaves normally .
Check the alt , reg , and ballast.