95 XJ idle stumbles thn revs up and idles fine?

Like the title says. Jeep idles perfectly smooth, then will start to stumble a little then 'catch' itself and rev up to 1100-1200 and then idle perfect again. I've tried searching with no real positive answers.

This happens when the motor is completely warmed up(maybe when it's cold too but I havent noticed)

Also it's throwing a code 12 and 21. I assume 21 is the O2 and code 12 is the PCM lose of power deal(How do you get rid of code 12?)

Thanks alot!

How many miles on that upstream oxygen sensor? Time for a new one most likely. I seek out NTK brand, which are the OEM supplier to Jeep.

The odds that you have a code (21) and symptoms are not likely a coincidence.

I believe it's original with 131k on it. I'm going to throw a new one in this weekend and see if it stops.

But what about this code 12? After 50 key starts will it just disappear?

Yes, don't be concerned about the code 12. More informational than related to an actual issue.