I have installed Kubuntu 11.04 recently. I used 10.04 before with KDE upgraded to 4.5.3(? last minor version of 4.5). There was no problem.
My laptop model is Inspiron 1520 with a Nvidia graphic card.
Here is a weird problem. When I maximize or minimize a window, or on every window resizing, whether desktop effects are on or of, screen flashes just tenth of a second and KWin freezes sometimes.
My Nvidia driver is NVIDIA-Linux-x86-270.41.06.run
The interesting issue is the problem don't occurred for GTK applications like Pidgin or Synaptic running on KDE.
Could you please how can I find the problem? I mean something like by means of a log file,a debug package or something like that.

Thanks in advance

If you switch from the Oxygen style to Plastique does this still occur?

the driver version 275.09.07 fix this problem