Lately the car has been running like crap. When i go to wot the car pulls fine till i get high in the rpm's, then it starts to dump black smoke and stumble. The only thing i have changed before it started doing this was i installed a mallory hi fire 6, a spintech muffler and a lt4 km. Also i installed a new opti not long after the ingnition box due to a miss through out the rpm's, that did fix that problem. New plugs and wires also. I watched the blm's and at wot the stay at 128 both sides. Any ideas what could cause this? no check engine lights other than egr malfunction. Any thoughts would be very appreciated. thanks.