I have a 2007 v6 convertible automatic, and have noticed the car occasional "bucks", particularly when driving uphill with constant throttle. It sometimes also occurs when accelerating in OD at low rpm. The car jerks back and hesitates for a split second, along with a "clunk" from beneath the car. It only happens once per time, and not in succession. I don't really notice the RPM changing when it happens, though I can't be sure. It's been pretty sporadic over the last several months (maybe once every week), but it seems to be more frequent now. My fear is it's the transmission, since the car is at 120,000. Is that the likely culprit, or could it be something less painful to repair? The car had it's 100,000 mile tune up, so it has new wires, plugs, and a fuel filter, so that's not likely the culprit.

 

Next time you go up that hill take it out of OD first and see if you get the same results. I am thinking there is a hang up in downshift. Mine took a dump at 117,000