I also get occasional complete crashes of the X server, about once every 2 weeks (average). I guess that's a different bug, but they might be related. It is very annoying as all applications are killed instantly.
When that happens, it tries to restart X and that always fails - so eventually it stops trying, and offers to run in "low graphics mode". However, once, it didn't even do that, instead it had the appearance of repeatedly trying to restart X forever.
That suggests the hardware, or the kernel, can get into a state where the X driver cannot reinitialise it.
I have checked whether booting with/without KMS ("nomodeset" to the kernel) makes a difference. It certainly affects the bootup screen (it's obviously not been tested much with nomodeset ;-) ), but:
The X display issues - this block pixel corruption, and occasional crashes - are the same with and without KMS. So it can probably be ruled out as the cause.