Wanted to give thanks to David, Christine, and daddi.

For anyone that comes across this thread searching, for me it was the kickdown switch. Took the switch off, sprayed some contact cleaner in/around it as best I could and pressed it by hand a few dozen times. Since then I've driven the car about a week (~400 miles) in all temps, speeds, distances etc and had no problems. I think the biggest key to if you're doing your own diagnostics was that it wasn't slow to upshift, it simply didn't until you hit the switch.