Help!

I have a 99 xj. about a minute after it is started it starts idling weird. It will drop down to 300 rpm(sounds like it's gonna stall) then surges to around 1200 rpm. Also if I try to drive it when it does this it feels like im patting the gas, off on off on. It doesn't really matter if its warmed up or not. It is worse if i drive a while then let it sit for an hour or so. Oh yeah there's a muffled popping sound that I assume is coming from the airbox but I can't tell cuase it only happens when i'm driving. It always stops after a couple of minutes, or if I FLOOR it. Any Ideas? I've checked the air filter and exhuast. And its got 72000 miles.

(1) Have you hooked it up to a diagnostic reader to see if it's putting out any error codes?

(2) At 72,000 miles you are approaching the end of the usable life for the oxygen sensor ... which controls the fuel/air ratio feeding the engine. I'm guessing that you may need a new O2 sensor.