
I'm having exactly the same problem.  Again, it's fine when I switch to 16
colors or a smaller monitor.  My configuration is:

Model: Centris 610 with 4 MB/80 HD, 512 VRAM, no cards
Monitor: MAG MX15F with 16" monitor adaptor (for resolution of 832*624)

I just discovered the problem a little while ago after plugging in my
new MAG monitor.  It seems to appear either when scrolling through a
window or when using Alpha or Word and I enter <return>.

My guess is bad VRAMs as well.  I really hope it isn't a design flaw.  Is
anyone at Apple listening?