Choppy Playback in Timeline, but not in Source Window

[CitizenSam]

Hi,

I am having an issue with Premiere on my Late 2013 Mac Pro.

Playback of footage is fine in the Source Monitor, but choppy in the Program Monitor when playing down from the timeline.

I have had this issue across dozens of projetcs with all different video formats, frame rates, etc. Even ProRes Proxies. We have seen the issue on Adobe 2020 and Adobe 2021 (and I think 2019). The exact same projects (with the same timeline settings) work fine on all our iMacs. It's just this Mac Pro trashcan that has the issue 100% of the time.

My company wants to throw the computer out, but since this issue is so specific to Premiere and so easily replicable, I'm wondering what can be done.

I can provide more information but again, we've seen this issue so broadly that I'm wondering if there's a known fix.