Rough idle

I have a 92 - 2 wheel drive 6 cyl. Explorer with 180,000 miles.
Started the engine when warm, heard a backfire in the engine compartment. The car ran fine but would barely idle. CHecked under the hood and found a vaccum line off on the intake manifold which I plugged back in.
Now the car will idle fine for about 30 seconds, then it will run rough with the idle jumping between 500 -1000 RPM. Otherwise the car runs fine. No water in oil, no leaks, no other noise.
Anyone have any idea where to start to look?

Thanks

I have a 97 and mine will idle just below 500 RPMs sometimes and then other times it is fine...so i know what your feelin.

And was that hose oily? Backfire and a hose off often occur when the vacuum modulator on the transmission starts leaking. Tranny fluid level down? I would clean the IAC on the side of the throttle body and also clean the MAF sensor at the air cleaner.