Hi Anne et al.
It's odds-on that acceleration method is the key. Intel and ATi are both rapidly developing their drivers to meet the more stringent requirements of recent distros. Kde4 desktops in particular uses parts of the driver not stressed in older versions. On this laptop I have an ATi display. The desktop works best with AccelMode set to EXA, but kuickshow has corrupted display in EXA and looks fine with XAA. It really does seem to be something where you just have to experiment.
You might find http://userbase.kde.org/GPU-Performance helpful
since I've switched AccelMode to XAA on my latop with the old i830 chipset, I didn't see any graphics corruption. I don't use desktop effects on this machine anyway, so I'm not missing out on any bling features :-)
Cheers,
Andreas