Very strange that only the 's' is changed to an 'x', generally stuff like this is a memory issue, but incorrect bits from the card's memory would suggest other characters being changed too. Maybe somehow the character table gets scrambled. Is it always this same substitution that happens?

Checking the card for anything out of the ordinary, like chip legs touching, seems like a good first step.

The 2nd image seems to show it better, but it looked like card was skipping every other row of pixels, giving a scan-line effect. I think the top and bottom parts of the "s" are cut off by the scanlines, making it look like an "x". A memory issue would be likely, but I've tested the memory with one of the programs on the Ultimate Boot CD and it passed. Games run without issues under Windows 9x.

Is this only occurring in text modes ?

If so, maybe it could be a corruption/weak bit(s) of the part of the video BIOS that contains fonts.

Does soft loading a VGA font into video RAM fix the issue when it appears ?

See https://int10h.org/blog/2019/05/fontraption-v … de-font-editor/

Also have you tried a different monitor?