Jeep 4.0 Misfire at 3500 R.P.M.

I hope somebody out there can help on this one.  I'm running a '91 Jeep 4.0 engine with it's electronics/harness in my '88 XJ.  Everything works fine except that it won't rev above 3500 rpm.  Once it hits 3500 it acts like cylinders are dropping off (it does it both sitting still and under load).  I've checked all the sensors for proper opperation (new O2, and injectors during swap, new crank sensor since), checked the wiring harness for continuity, checked all the grounds, all new ignition parts (cap, rotor, wires, plugs, coil), tried a couple of spare distributors (even went so far as to clip the ears off one to alter indexing to see if it would help), and a spare computer.  Checked the fuel pump for both preasure and flow.  Checked for vacuum leaks.  The problem is still there.  The computer doesn't show up any trouble codes.  The problem doesn't affect daily driving, but it would be nice to rev the engine a bit more when in the mud.
Anybody out there that has any idea what the problem is or might be PLEASE let me know!!!!  

What do the plugs look like after heavy misfiring?

Plugs looked O.K. when I took them out.  Just a little carbon on #6 No heavy build-up of deposits or oil.  

#6 is your problem child then. Carbon tends to suggest that mixture is going rich in that jug and causing the misfire. Double check wiring and dist cap. Run a compression check too and see what you find.

Wires and cap are fine.  Compression is about 130 across the board.  I think the problem is somewhere in the electronics as it was doing this with the first H.O. I put in and also with a '96 long block I installed after the first engine blew a rod bearing.  I double checked the crank sensor, cam sensor, throttle sensor and MAP sensor with an multi-meter and everything is switching like they should.  

I think you need to focus on the injection system and possible fuel pressure too at higher flow rates as engine revs up.

Sounds like the rev limiter in the ECM is engaging at 3500 rpm.  

Very good point because it is was a 4cyl ECM originaly, they would mean about 5250 RPM cut out which would be very realistic.

Acting like a rev limiter definately describes what's going on best.  I've tried two different ECMs (original '91 computer and another used one from a '92 both are for 4.0) with the same results.  I have another one from a 4.0 manual on the way which I'll try.  

I have the same exact problem with my 95 and i tried the same exact steps as well.  I am going to recheck all the dist. components and the alignment and then maybe the cps that is all i can think of.  I know what you mean and if you find a solution please let me know.