20 year old XJ being cranky, inttermittant high/low Idle

I'm really not sure whats doing this but my 87 Jeep is idleing like it has a mind of its own. Usually from a cold start on a warm day, it revs right up to around 1500-2000 rpms, this is on start up. At other times, usually after I have been driving it for a little while and a few other times usually on damp days, it idles really low, usually I have had it idleing at around 750 pretty smooth, IIRC the 4.0 should idle at 500rpms, mine will go to about 450-500 but it will be missing like crazy almost to the point its going to stall. I am thinking this could be one of the sensors, all my ignition stuff is pretty new (cap, rotor, plugs and wires) and I'm running champions by the way. Another thing, where is a good source of replacement sensors, cause chryco dosn't stock most of the renix stuff now.

My '88 used to idle really high when cold like around 2000 rpm. I changed the TPS and that did the trick.

you can get most of the renix stuff still at any of the chain stores.

your idle problem is likely caused by an out of adjustment TPS, a failing TPS, or a lazy IAC.
Adjust the TPS first, it's free, and while you're adjusting it you can check it for dead spots with the meter.
If the TPS checks out fine, removed the IAC from the throttle body and clean it's bore. DO NOT move the IAC's pintle while it is out of the throttle body.