I have a 95 Z28 M6 with 151,000 miles on it. It has been very dependable, and has run great until a couple months ago when it started sputtering. At first I noticed it at WOT in 5th and 6th gears. It would buck and shake the car. Now it runs really "rough" when first starting up, and falls flat on its face in upper RPMs, and the bucking and shaking have gotten worse. After it warms up though, say after about 10 minutes of driving, it seems to run *much* better, though it still stumbles at WOT in upper gears.
I have already replaced the fuel filter and spark plugs. I took a look at the spark plug wires and connections, and they look great and I haven`t noticed any arching. I also ran fuel injector cleaner through it, which made no difference. The air filter is recent as well, and there are no leaks (that I can tell anyway) after the MAF sensor. Any ideas?

 

Try disconnecting the cats and see what happens. Also, see what's going on at the EGR. Check it for movement by manually applying vacuum to the hose.

 

Thanks slop. It actually developed into what feels like a full blown miss now, when its hot or cold.

 

It sounds similar to whats happening to my car with 107K miles. I changed plugs, miss still there. I did a compression check, and all looks fine. My plug wires are ovtvc, and they look great. I went out last night in the dark and checked for any arcing, and it was hard to see, but 5 of the 8 are arcing to the valve covers. Very hard to see, but there. Don't be in denial about your plug wires. I ordered a new set, and should be here in a couple of days. This isn't the first time this has happenned either, and every other time for me atleast, it was plug wires.

Z