1990 runs rough

I just sold a 90 Xj with the 4.0 auto. i sold it to a kid that works for me and feel kind of obligated to help him figure this out. it ran fine thursday when he picked it up and all day friday. all of the sudden friday night it had no power and idles really rough. with no load on the motor if you feather the gas and get it out of the rough idle it revs to the moon. i have been reading the forums all day, but none of the symptoms seem to match just right. it kinda runs like my 92 does when it is below 1/4 tank, but when ever mine does it i shut it off let it sit and then its fine(i just keep the tank full now. his has 3/4 tank, sat overnight and still crappy. i just changed the plugs the day before he picked it up. any ideas???

TPS maybe? no power and idles really rough. with no load on the motor if you feather the gas and get it out of the rough idle it revs to the moon.

Always have to verify tuneup hardware when you have driveability symptoms.
That includes plugs, plug wires, distributor cap, rotor, fuel filter, EGR valve operation.

After that, it's time to start testing things like fuel pressure and individual sensors to try to isolate the problem.

In this case definitely check EGR valve operation, this will cause a rough idle. TPS, IAC, and as said above tune up. Did you gap the plugs correctly?