[Alex Gerulaitis] “The display driver has a 720×480 preset? (Usually the lowest is 640×480. 720×480 is DV signal resolution, haven’t heard of it on a display monitor.)”
The Nvidia control panel allowed me to force certain custom resolutions, which I did to test out your suggestion as thoroughly as possible. It’s definitely not a resolution I would use regularly!
So I decided to go nuclear and completely re-install the OS and programs from scratch, checking the playback in Premiere after each driver/program install in an effort to weed out the issue.
The good news is that everything works as it should now (so definitely not a hardware issue). Smooth playback with no issues to report.
Maybe my system just got bogged down by some crappy update or driver that I didn’t notice at the time, or the registry got too messy. Hopefully the problem won’t come back, but if it does, I’ll try to pay closer attention to what might have caused it and post the solution!
Thanks to everyone who offered ideas!