strange back fire

hey yall,i was driving my fiero the other night and it was running good but,when i left where i was at and i hit the gas @ 60 mph it would back fire through the exhaust,i have a d.i.s and it says in the book that the system actually fires on the exhaust stroke at low voltage and the comp.stroke at hi voltage now after i read this is sounded plausable even with my problem except it almost sounds like the exhaust stroke is firing also can anyone help out with this is there somthing i can change to tkae care of this problem thanks ke4mns