I'm having the same, very annoying, problems here with Mint KDE 64 RC!
For me often occurs using Firefox, while creating or switching tabs.
I've tried to disable blur transition, but still happens.
I've just tried to disable all desktop effects (Alt+Upper case+F12) and I'll let you know if it's, as I think, it's an effect related problem.
Linux Mint 64 KDE RC
AMD 64 X2 3800+
DFI LanParty UT NF4 Ultra-D
4 GB Corsair DDR 400 Dual Channel