X freezes in FC 18

[Jens Neu]

Dear list,

I'm experiencing random freezes of two desktops, both up-to-date FC 18, 
totally different hardware (Core2Duo E5300, Core i7), different Graphis 
Cards but both Nvidia using nouveau with a dual monitor setup.

Desktop completely freezes, Num Lock on keyboard is dead. Gut feeling 
tells me it is induced by scrolling.

When happening, I can ssh into the machine, everything looks perfectly 
normal. No 100% CPU on nothing, no zombies, no whatever. Nothing in 
dmesg, Xorg.0.log, messages.

Restart of graphical.target does not do anything, when killing 
/usr/bin/X (with -9 though) I get kdm back.

Any thoughts where to debug further?
regards
Jens