Rough at low RPM
02 Ranger Edge with 150,000 miles.
A few weeks ago my truck started idling rough. Not all the time, but at a light it would usually have a few rough patches. Then it started doing it while driving, but only at low RPM. Typically if I come down a hill at about 50 mph, and then start going up, it will hesitate. When it downshifts and the RPMs go up, it smooths out. Seems to have plenty of power at higher RPMs.
I've gone through two tanks of gas, and replaced the plugs (they were getting old anyway).
Yesterday I got a check engine light and a misfire on 1, but that's only happened once.
It's worse with the AC on.
Any ideas what's wrong?