I'm thinking dist pickup coil/ECU or maybe a loose connection which might be a bear to track down. You are fine on the 2 terminal ballast. Your parts house has 4 terminal ECU's for cheap. I dont like to shotgun parts at a problem but carrying a spare ECU is always a good idea (if it turns out that ain't it) if you cant borrow one to do a check. Nothing was changed right prior to this happening?