surging ranger

[dieselade]

my 2001 ranger 4x4 has a ghost....when the ac is on and I stop and hold brake for a stop sign, it feels like someone is behind my ranger and trying to push it or is pushing up and down on the rear bumper. the rpm drops and recovers repeatably. it seems to be ok with the ac turned off. any suggestions. not throwing a check engine light.

 

[srteach]

the surge is probably the engine trying to maintain a specific rpm to power the AC clutch. This could mean a bad O2, partially blocked intake, or a sensor somewhere else. I had a surge problem that went away after an O2 change.
First, full tune up. Then plug a scanner into it to see if any of the sensors may be close to limits. After that, I'd be guessing, too.

BTW, I assume an auto trans, right?