Updated the Wacom tablet drivers and ran Disk Warrior on the hard drive, which seems to have locked the problem down. Curiouser and curiouser: before I did the repairs/updates, several Photoshop files came up "file damaged" with odd color bands in the art when I opened them. Cue much screaming from me, followed by closing of the files and repairing/updating. Lo and behold, when I reopened the files afterwards, they were PERFECTLY FINE. No "file damaged" warning, color bands gone.

Holy fuck. I hate it when computers go wiggy. I hate it even more when errors fix themselves for no discernable reason.