99 Malibu - transmission slips on first shift

99 Malibu 272K original engine/transmission - Firstly, this is not an issue related to the mileage, this started at around 60K and has never changed one bit. I'm only curious as I'm gettting rid of this one soon (regrettably) and it will sell better if I can fix this as the car still is in great shape. When and only when the car is first put into gear, it seems as though the torque converter slips some until you get the rpms above 2500 or the car gets rolling (like downhill.) After the first upshift, never a hint of the issue unless you shift the transmission back out of drive. Doesn't matter hot or cold, you can drive it 1000 miles, come to a stop, shift to park then drive and it will happen. But, it has always been intermittent. It will do it every time for a week, then maybe not at all for a month. Fluid level is fine and fluid has always looked/smelled normal. Tried some stop slip products over the years with no effect. Read somewhere once that a very similar issue in some impalas was actually a programming issue.