Since a couple of days I experience some strange graphical artefacts on Snow Leopard. Usually after a certain uptime, the background image would show some strange colours at some place. This could be reverted by reloading the background image. But the problem would appear again after rebooting (and waiting for an indefinite time).

Sometimes, the shadow of windows would also have some artefacts; and this time the application switcher is completely distorted.

Any ideas where I need to search for a solution of the problem?

Edit: Model: MacBook Gen. 3.1 / all updates installed

Update: After not having used Safari for a while, the problem has not occurred anymore (at least not after 20 days of uptime). Could well be that the problem had its origin in Safari or a Safari-related plugin. (Possibly Flash or Click-to-Flash as it did not happen with both deactivated, though the running time with this configuration might have been to short to be of any validity.)