Idle problems again

I know this problem has been addressed many times on the various Jeep
boards, but I have tried the suggestions and am still having problems.
My XJ runs fine for a time then begins to idle high. I can kick the
throttle with no dropping back of the idle. If I shut off the engine
for approx 5 seconds, the idle returns to normal, then minutes later
is high again.
I have replaced the IAC valve and the TPS sensor, MAP sensor and now
a full throttle body with IAC and TPS from donor parts Jeep. No help.
The donor runs perfectly, with no idle problems, so now I know it is
not the IAC or the TPS. I have had this problem in the past, replaced
the IAC and problem solved. This time, though, no solution. We are
stuck and can't figure out what to try next.
Thanks for any suggestions we might be able to try.

what year XJ? how does the vacume line to the fuel pressure regulater look?

It's an '89 with 4.0, automatic. It's down the street at a friends house where we have been pulling parts off his XJ to try to find the problem, so I have to get down there to check the vacume line. As soon as I can, I'll respond. Offhand, I'd say the line is probably good, as I had a mechanic replace all PCV and vacume lines last summer. No guarantee, though, because we found a bad line from the the back of the TB yesterday. He's checking for vacume leaks today.
Thanks for the help.

just an idea. I know when you remove that line the fuel pressure goes up and the idle increases. if you've tried the usual tps cps and slaved in a ECU Id make sure you have good vacume there and maybe try an O2 sensor.

Ive also seen a sticky throttle cable cause this too

I had an intermittant high idle on my 88, cleaned the grounds from the battery to the dipstick brace and also the ground to the firewall on the drivers side. Seemed to help. Even if it doesn´t help, cleaning the grounds, is always good idea, sure helped mine idle smoother.
On the wifes old 87, had a high idle, loose intake manifold bolts. As soon as it would begin to warm up, a bit ,it would idle high.
Had another, the front harness, the extension from the fuel rail (injector) harness, that runs down the front of the motor and underneath, had flopped over onto the exhaust manifold. Was shorting the temperature sensor, for the computer (lower left of block). Caused high idle.
Turning the heater controls to the defrost position (with an A/C equiped model) will up the idle some.
Another spot to check for vacume leaks, is under the battery, the major vacume line, that goes to the vacume resivoir, mounted behind the bumper. Often messes up the heater controls, I´d guess it could also affect idle.
Check the MAF wires, they flop down on the exhaust manifiold sometimes, could cause idle problems.
Been there done that, good luck

What about the vacuum lines that go to the front axle?