ive had this problem before... the car would knock and stumble and skip, backfire, rock, and do everything wrong... Someone said it was a wrist pin, crank, lifer, rod.. it sounded so bad.. .like the engine was going to shake and distroy itself to death....and my god was it loud. It litterally made the chevy mechanics at the dealership go.. "my god" and shake their heads.. It turned out to be the cap and rotor... It was sending a spark to some of the cylnders early.. so it would fire with an intake/exuast valve still open... I Love opti-sparks.
.. I replaced it.. and a week later its doing it again