Slow Motion glitchy after render and encoding in media encoder

hi. First off, I apologize if any of my terminology is off, as I am self-taught and use Premiere Pro to edit videos for YouTube (aka, not a pro). I've been using it for a year andd a half, but in the last few months (after an update I installed over the summer) I've had a consistent problem: clips are glitching in my final output, despite appearing fine in my editing timeline. I am using clips from a few different cameras and shooting at different framerates (30, 60, and 120, depending on the camera and intended output of the clip; cameras include a Sony, GoPro, iPhone, and Canon 5D MkIV). I am interpreting the footage after import so that it is all at the same framerate as the sequence (29.97), rendering and replacing any clips that I warp stabilize or change the time on (usually slow motion, occasionally speed up). When I export the final edit I select "Use Previews" (and I make sure the setting in PP are for high quality previews to begin with and no frames are being dropped in playback). When the video has finished encoding and I watch it back, there are inevitably 2-5 clips that glitch. So I go back into PP, fiddle with them or replace them, then export again - this time another 2-5 clips will glitch. But those clips were fine in the first export and I didn't make any adjustments to them before exporting again. All of the glitchy clips have speed adjustment or warp stabilizer applied to them. Other than that, I've been completely unable to predict what might cause it to happen and to which clips (as there are dozens in each video). Is this possibly a memory issue? I am using a 2020 MacBook Pro with M1 and max RAM. Thanks for any help solving this. 

(I should add that I recently had to make a small change to a video I edited last year - I edited it last on a 2013 MacBook Pro that was hopelessly slow, but it encoded last year with no glitches. Today on the newer machine and PP version, I removed the last 2 clips (otherwise leaving the rest as it was) and exported it, and this time I had multiple glitches on various clips throughout the video.)