94 z28 a4 110,000
a few days ago my car started this weird thing were it would backfire at high rpms but then after my car cooled down it ran fine. slowly after a few days it got worse and worse and now i only have about 10 minutes of runtime and it takes a crap and starts missing real bad and backfiring. i replaced igniton coil 5 min ago and it didnt fix it. Opti only has about 10,000 on it and all new plugs and wires...could it be bad o2's or the module on the ignition coil.

 

My old engine on my 1994 Z28 had 120,000 miles on it before it started a valve seal leak and started fouling the plugs with oil. The hotter it got, the worse it got. Have you checked the spark plugs to see if they are gunked up??? If you are getting less and less drive time, then it would seem reasonable that a possible leak from the valve seals would foul the plugs more and more. It would also account for why all of your electrical parts that have been replaced haven't fixed the issue. I would see what the spark plugs look like since you replaced everything else. Just make sure to chuck all 8 plugs. That was the issue I had with my old block. Hope that helps.