Re: Low RPM's
Get a cheap voltmeter, and measure directly to the battery posts, then turn on the sub and A/C separately and see how much of a drop you get.
Separately, measure across the two posts, then measure from the + to the body right near the battery and see if there is a difference. You could have a bad ground.
That would be the easy stuff, after than you try to pull codes and see if the CPS is having a bad day, O2 sensors, plugged cat...
Wonder why the idle is so high on your 4.0... never hit 1000 even with A/C engaged.