I thought of a vacuum leak also, but I would think you would get lean or rich code, and it would show up at idle.
I still keep coming back to the fact that it only happens in gear. Maybe there is a vacuum line attached to the transmission that is leaking only when the drive train is in certain positions. I'm grasping here, but I'd at least look for vacuum/transmission related things. One thing to check is that your dip stick for the transmission is pushed all the way in (assuming you have a ATX).
Yep, I have almost 149,000 now. Pretty much uneventful - I've been blessed to have such a reliable vehicle but I believe that proper maintenance schedules plays a big part.
