There don't seem to be vacuum leaks, vacuum looks good.
Catalytic converter is a non-issue, my cats are hollowed out so there is no way for blockage there.

I did some sensor graphs with my OBDII scan tool. Throttle position sensor looks good, MAF looks good, O2 sensor readings look good.

I checked short term and long term fuel trim, they get up to around 5% at times. I don't know if this is high or not.

The problem seems to be getting worse. Now it stumbles even while idling.

Disconnecting the idle air control valve doesn't seem to affect this, so I don't think the IAC is the problem.

More and more I am suspecting either fuel pressure supply problems OR... spark plug / coil / wire issues or arcing.

One thing I recall in the past was that the rearmost spark plug hole had a bunch of oil leaked into it because the valve cover seal had leaked oil into the spark plug well. Would oil in there with the spark plug wire cause problems? Does oil conduct electricity?

I will try to check the spark plugs and wires / coils tonight or tomorrow night.

2002 Sportage 4dr 4x4 soon to undergo an Extreme Makeover!