So I have a pretty uncommon car, a 1998 Oldsmobile LSS with the 3800 Supercharged and 4T65E-HD. I'm trying to figure out what is going on with the transmission, it has some random slipping. I first noticed the slipping going up a very steep hill one day, noticed the engine was revving but I wasn't pulling the hill as well as it should. The slip isn't bad, but if I take off about half throttle sometimes it will slip, the car will move forward slowly, the engine will rev to about 3000 then catch and drop back down and take off perfectly. For some reason it does this more frequently on right hand, slightly sloped turns. No slips full throttle, and all gears shift crisp. However, on light throttle when the transmission downshifts from 4th to 3rd, it will slip for a second and then jerk slightly and catch 3rd. However its only at light-medium throttle, full throttle downshifts are crisp and firm.

I've added the TransGo 1-2 and 2-3 accumulator springs/spacers, and a decently sized stacked plate trans cooler, did a pan drop/fill twice using a WIX transmission filter, and tried Castrol Dex/Merc and Valvoline MaxLife high mileage Dex/Merc and there has been no change. Any ideas? Thanks!

How many miles are on the transmission? When hot in park after driving for 20 minutes, are you on the full line? Did you notice any clutch or fine particulate matter in pan?
It may need rebuilding if it start to slip lots.
Dexron VI (6) is what I would recommend. It'* synthetic and may help to keep things cooler as well and is the latest recommended trans fluid.
Just in case it is a sticking pressure solenoid, you could try adding a half can or full, depending how much room to full line (or slightly above) on the dipstick you have left, of Seafoam Trans-Tune.
That may get some varnish out of valve body and solenoids, and may shift better quickly after adding it.

Seems to me, I remember, a TSB out there, describing a "phantom neutral"....your description sounds like this....