This problem (as zuluhead has described) has been described to me as torque converter shudder. I don't know exactly what that entails, but I am fairly sure that replacing the torque converter is part of the repair. Don't quote me on that though.
It is quite possible that by only doing a pan drop on your car (which only replaces from 30-50% of the fluid), that the rest of the old fluid that is in there is still causing the problem. Many people do not like transmission flushes because of their ability to work loose crud in the engine that may be keeping things sealed up. If you do not feel comfortable with a flush, then maybe 2 pan drops would be a better choice (or 1 pan drop to replace the filter, and a suction line down the dip-stick tube to suck out the fluid after you have driven it for 200-400 miles).
On a related note, I had this same issue with mine last year. I haven't experienced it yet this year, but when I was having the issue, if I were to tap the brakes, it would completely go away (which is why it led me to the TC issue - when you apply the brakes, the TC will unlock). I also think that I had gotten some bad gas that might have made matters seem worse than they actually were.