Playback On Second Monitor Stuttering/Slow

[KTOlmsted]

Hello all, long time listener, first time caller here...

I work in local TV news promotions/creative services, and I'm working at home right now. Our station wasn't prepared for this, so I'm working with all of my own personal gear. I encountered a weird problem today-

My set-up - I am running Ae (as well as Pr, Ps & Ai) 2020 on a Surface Book 2 with 8Gb of RAM. All projects are running off a 1Tb external SSD. I am running a second monitor (1080p 23.5" ViewSonic) with a USB-C to HDMI cable. Everything runs surprisingly well - video plays smoothy to the second monitor from Pr and my previews run just as well in Ae. I am NOT a Windows guy. I've been a Mac guy since Apple IIGS. My old Mac crapped out and I thought I'd give Windows a try to get familiar with it, if anything for job security. By the way, a new Mac is on it's way.

My problem - Starting this afternoon, when I do a RAM preview in AeE, the playback (in the second monitor) stutters. Or rather, it plays back at half-time. That's the best way to describe it.  It looks like it's playing slower with a reduced framerate. When I go back to single-screen mode, it plays fine in the laptop monitor. For what it's worth, I tested Pr and it still plays back perfectly in the second monitor.

What changed - I was Zooming with my boss early this morning and we were trying to figure out if I could screenshare Ae playback good enough for animation concept approval. I set Ae's UI to single screen mode so everything was in the main monitor. He didn't care for the playback through Zoom, so we abandoned it. It was only after I went back to dual-screen mode in Ae that I started having this playback issue.

What I tried - Clearing all AE caches, resetting AE preferences, resetting my computer and updating AE through CC. I opened older projects to make sure it wasn't a project-specific thing and all older projects suffer the same playback issue now as well.

Any help anyone can offer is appreciated. I don't know Windows well enough to troubleshoot this very deeply, if this is a Windows thing at all.

[KTOlmsted]

Update - This morning I completely deleted After Effects and all preferences associated with it (using the uninstaller). I reinstalled the app, opened it and arranged my composition monitor in my second screen. I opened a project, previewed it, and encountered the same problem.