PR CC V 14.0 2021: Some clips suddenly do the "Jitter Dance"

Hi - I'm relatively new to Pr (maybe 1 year in) but doing just fine; in working on a client project, I noticed most clips render fine on export, but inevitably 1 or 2 clips do the "jitter dance", meaning, the video is not smooth and frames seem to bounce backward and forward. What's odd is some of these jittering clips rendered great on first export, THEN, on future exports same clips do the "jitter". I've searched why this might be and did what the search results suggested, including allocating less ram to "other applications" (more for Pr), clearing cache, etc. Best explanation is "source video files have become corrupt", so I replace the clip "fresh" from the source and it's fixed, then, a second export it does the "jitter" again, as though the new clip became corrupt. I'm pre-rendering when I can prior to final export, learning about and trying different Codecs and Sequence Settings. Jitter dance still occurs. I assumed using Pr CC on a paid account, this sort of thing wouldn't occur, that my exports would be clean and beautiful. 6th hour now "chasing this jitter issue" when the fall weather is beautiful outside.

Running on Mac OS High Sierra on iMac 27" 2011, 2.4 Ghz Intel Core i7; 16 gigs RAM; Graphics AMD Radeon HD 512mb; I just noticed today via Creative Cloud that the new Pr v15 "is not compatible". See GIF attached for "jitter". - thanks, #NoMoreJitters