Having recently lost a transmission to old age, I know the feeling. A lot of things can cause something like this, and low fluid pressure (like from a clogging transmission filter) is the cheapest one to address. Drop the pan, replace the filter, reassemble, refill with Dexron-VI.
If you're having internal transmission wear or damage, you'll probably see it in the form of dark or burnt fluid, tiny metal bits, gunk, or large pieces of whoknowswhat in the bottom of the pan.
If everything looks good, the new fluid and filter should help. My last 4T60 wasn't staying firmly in overdrive before I serviced it. After that, I got a smooth 75,000 more miles out of it.
2007 Pontiac G5
1999 Pontiac Grand Prix GT - Stock engine ate transmission.
1997 Pontiac Bonneville SE - Cammed engine ate transmission.