Mileage is about 165,000. After things warm up the transmission will not stay in OD/4th, but seems to drop back to 3rd, then back to 4th - about 250 rpm difference - and keep repeating that if I don't switch of O/D. When it started happening it was just under light throttle - give it some gas and everything would be solid. But it is getting worse. Sometimes, very occasionally, it will just go into neutral until I punch it. At speed ( 70-80 mph) no issues (yet).

I have had the common snap ring problem, and fixed that maybe 2 years ago. ( Thanks for all help on that one!), and I know what that was like - this is different.

Any ideas? Fluid level is fine - maybe even slightly too full?