Right now, it's not happening after extended play sessions -- or, at least, it's not worse after playing for a while (suggesting overheating). I haven't had any lockups, either. The problem is picky about where it'll show up; it's only certain games, at certain times in those games. (For example, many stages in Soul Calibur are completely glitch-free, and I see no scratches or marks on my disc.) Of all my games, Evil Dead is the only one with a major problem -- and, quite frankly, it's a sufficiently terrible game that I'm not weeping over the loss.
In particular, Evil Dead is the only game where the lines of dots are bright green. They've been almost clear and hard to notice when appearing in other games. It could be that Evil Dead uses a graphics mode or resolution that's unusual. Even in that game, some of the screens in the FMV are clear, but they always come back in the title screen and gameplay.
I'm not a mechanical type, so I suppose I'll see if it gets worse and grab a $69.99 used DC from my local Electronics Boutique if it does.