This is very strange.....

Car details: 1994 M6 Z28 Vert 130k miles

When driving full throttle 1st through 4th the car runs outstanding. Once I hit 5th gear it stumbles and misfires badly! It continues into 6th gear. If I go back to 4th after slowing down it will accelerate 100% again.

Any thoughts on this?

 

What are your mods? Any wires arcing? I have headers and just fixed this exact problem yesterday. Does your SES light flash at you once you're in 5th?

 

The car does have headers but I just had it at the mechanic and he fixed some loose connections but the plug wires were fine.

The SES light is on all the time due to a o2 sensor but no it never flashes.

 

So if the plug wires are probably fine, then that leaves you with what to cause a misfire? Opti or bad coil? Have those been checked out? Timing?

 

It appears I have a fuel cutoff at 115mph. or thats my speed limiter?

 

i believe a bad o2 can cause that problem....