sounds like catalayic converter could be bad, if u can get under there and hit it or shake it and listen for stuff rattleing inside then u kno its it, also i had a 93 with a bad crank sensor wire, it was shorting out together and o man it would back fire. took me a long time to find that short but eventually did, and i think i got the wire from the junk yard for like a dollar, the owner was happy as hell and still driving it, been over a year. hope this helps