VSE - Clip is glitching?

And I don’t know why! Agggghhhhhh!!! See clip, I screen-recorded myself playing the VSE alongside the source video in question, which is playing via QuickTime Player. As you can see there’s nothing wrong with the source, but the clip in the VSE keeps stopping and re-looping from the beginning (where the model’s leg flexes, if you can see that).

This happens with every clip I add these days if I put some kind of transform/modifier on it. For this clip, all I did was image offset and resized the render resolution to make the clip more square than iphone-sized, and as soon as I did that it starts glitching like this. Removing the offset/render size change makes it play fine again.

And no, this isn’t a proxy issue to my knowledge. No, I’m not using any proxies on this short clip, but…When I render the animation in any frame range, it renders with this looping glitch. And I’ve tried to make proxies, but the proxies glitch even worse for some reason. (As a matter of fact, they don’t even play. Just stay frozen on the first frame.) Memory cache limit is at 8000 if that matters, my max is 32000 and raising it changes nothing.

Possibly unrelated matter: Every time I try to render as a video, I get “Could not initialize streams, probably unsupported codec combination” every time unless I export as AVI/jpeg. No combination of anything else works.

I just don’t understand why all this is happening; I’ve used the Blender VSE before—like, within the past year—and none of this happened; sure, adding modifiers and transformations and stuff slowed down the in-window playback speed, but it didn’t cause problems like this. Also the codec message is new. I just don’t know what’s going on. SOS

EDIT: Now it keeps glitching no matter what I do, even with no offset and no render size change. Welp.

Ok, I’ll bite. What’s your input format?

The source is a 1920x1080 clip from an iPhone in H.264 AAC, saved as a .mov file.

The update is: I scrapped the project and started again with the same clips, …and… now it’s working fine, and not glitching when I crop/transform anything. I’m still stumped as to why this was happening but for now am blazing onwards.

Also in the old project, try making proxies and use those. Then render out, does it exhibit the glitching still?