Just to be clear, the aforementioned behavior happens very infrequently, at least to me.
Usually I came across screen artifacts on Aquaria (always), on some Cherry Darling's games and even less on QIII, AvP. On OWB sometimes when there is a doodle on Google homepage (but if I iconify the browser they go away).
Regarding slow down on boot, when Mark provided me with an altered driver that he disabled something, it has been working correctly for many years since then.
I would suggest you get in direct contact with Mark since it sounds to me like the driver issue I faced as well in the past. But at least I know that my card was flashed and I took my chances with it :P