OK, before anyone jumps to club me just listen.. i know it may sound stupid, but check your voltage( battery/alternator).. my alternator is putting out VERY low voltage, but since im broke as crap right now i just stick it on a charger every 2-4 days. About 2 weeks ago i was trying to make it home as it was dying and for about 3-4 minutes before pulling in the driveway it was back firing like crazy. but only under hard throttle.. I doubt this is your problem, but always good to eliminate easy things first. do you still have your old wires? i would put them back on and see if the problem goes away. if so get another set of wires..