Chuggle?? Surge...05 Taurus

[Bird Dog]

First I'd like to thank in advance for any info you can give me...were better to look for info on fords than a ford forum.

Just bought a 05 Taurus 3.0 duratec 89K. Nice car, runs nice except...First noticed the "problem" the other day, driving in town 30mph, 1600rpm. The car would lightly buck, surge, chug, etc. Can barely feel it but can feel it non the less. I changed the plugs/wires, sprayed out the TPS and throttle body. No change. Then my wife tells me it was doing it more today, first warm weather of the season, she had the AC on. SO I took it for a test run. With the AC or fan on the car will "chug"...lightly at most steady speeds. Shut the AC/fan off, it'll only do it at the low 30mph speed. Where to start looking, what might be the issue??

Thanks...