"WRITE ERROR" and weird bad spots for movies on X-T4

[Astigmatism]

I'm just now trying to learn about movies, on an X-T4 I've been using for still photography for a few months now. But most of the movies I try to shoot give me a "WRITE ERROR" during "STORING", and leave nothing on the card. Sometimes I get a movie on the card in spite of the error message. Some of the movies I do get have weird spots in them, maybe the whole screen mostly goes a single color with some different looking areas here and there. I've experimented with different movie modes, formatting the card in the camera, and power cycling the camera, but have not found a way to make most of the movies work. I'm using SanDisk Extreme PRO SDXC 128 GB cards that I bought specifically because something from Fuji recommended them, and not using them for anything else. The cards go back and forth between my cameras (I also have an X-T30 ii) and my iMac.

I haven't had any problem like this when doing still photography, or when using the cards to do camera body or lens firmware updates (which I think are all current).

Any suggestions for how I can figure out whatever's going wrong?

 

Thank you!

[Astigmatism]

The cards I have are 170 MB/s, so I ordered a 300 from the list elsewhere on Fuji X Forum. Bet that fixes it.