2003 Pontiac Bonneville Transmission pops out of gear @ slo

I sold a friend this 03 Bonneville with 70k mikes on it. It now has 91k. Recently he describes (and I felt it also) when accelerating from a stop it sometimes seems to "pop out of gear".
The engine revs but no power is being put out thru the tranny. After a split second, it will pop back into gear and the car is being powered along once again. This also occurs when DE-accelerating- as when the car is stopping but again, only at very low speeds, under 15mph or so. I did a mini stall test with my foot on the brake and floored the accelerator for a few seconds and the clutches do not slip. The fluid is full and looks/smells fine. It has never been changed. Please help. Thanks Mike Ellis