video export is glitchy, distorted

When I export my video, certain moments will have a glitchy/distorted overlay on them, as if the video has struggled to process certain clips. What's frustrating is that when I reexport, it can be in a different spot and NOT in the same spot it was before. But alas, it's always SOMEwhere as the video i've made is 5 minutes so it seems impossible to luck out with 0 glitches... Here's a picture of what it these brief glitches look like:

It looks as though it's pulling a still image from an earlier part of the video (not a bordering clip). Sometimes this happens in the timeline when i'm watching before i export, but it goes away if i just pause and replay that section again.

This has never happened before and I used iMovie every other month or so.

Has anyone experienced this or know what I can do?

My specs are:

iMovie 10.1.14

macOS Mojave 10.14.6 (not going to upgrade to Catalina because many of my apps will stop working)

iMac Retina 5K, 27-inch 2017

4.2GHz Intel Core i7

24GB 2400 MHz DDR4 RAM

Radeon Pro 575 4 GB