BTW, ran Memtest for ~3 hours. No errors.

I know from experience that memory test programs do not always find bad RAM.


I'm considering replacing the video card, since it seems
a likely suspect.

If the card works under multiple operating systems, that pretty much rules
out the card. However, you might have problems with the driver.

One of the good things about cards using Nvidia chips is that they'll run
with "generic" drivers written by Nvidia. (This is also probably true of the
other major brand, whose name I can't think of). Try using one of these.