STUMPED! need help

i own a 1992 XJ 5 speed 2wd. I have the 4.0L, at high RPMs it seems to break up. there is no check engine light. scanner doesnt pick anything up. almost as if the timing is way advanced. but seeing as how it is electronically controlled i dont know how this could be. HELP! :huh:

What spark plugs are you running, what do you have them gapped at and are they new? Try the next cooler heat range....
Sounds as though they are missing at the higher combustion chamber temperatures and faster flame travel...

I would start with a good tuneup, if you haven't already.

Plugs, wires, cap, rotor, air filter, fuel filter.

Could also be a plugged cat or something of that nature.

How well does your spedometer work? I had issues with the speed sensor pegging out which made the computer engage the speed limiter even at lower speeds. Mine is a '92 also.