The upshift lamp is a straight shot (through a couple of connectors) from the ECM to the instrument cluster. The computer turns it on and off.
I don't really understand what you mean when you say "the engine powers out", charging system? The motor goes flat (no power, slow acceleration) ? Can you be a bit more descriptive?
Most of the sensors work off of the same 5 volt supply, if one sensor is shorting it can cause some strange things to happen.
CPS and Cam sync sensor are some prime trouble spots for 5 volt power supply shorts. A short in the 5 volt supply often causes the check engine lamp to not come on when the key is turned to the run position.
Just throwing something out there, that may or may not be relevant.
Naxja.org is a bunch of XJ guys that have pretty much seen it all. If you can post up there, you may get some more ideas. Some of these guys would welcome a problem they have never seen before to break the monotony of the same old porblems showing up again and again.