I just narrowed down a weird problem where I was losing more than 50% of my vblank interrupts to what seems to be the hires timers patch. Stock 2.6.23-rc5 works fine, but the latest (171) kernel from rawhide drops most of my interrupts unless I also have another interrupt source running (e.g. if I hold down a key or move the mouse I get the expected number of vblank interrupts, otherwise I get between 3 and 30 instead of the expected 60 per second).
Any ideas? It seems like it might be bad APIC programming, but I haven't gone through those mods to look for suspects...