(In reply to joro-2013 from comment #4)
> I've noticed the same difference on my ancient mac mini G4 with a humble
> RV280 GPU. I try to boot it in AGP mode from time to time after applying
> some patches, just for fun. Before i would see some GPU lockup-resetting
> stuff (it always locks up after some time and never recovers) in kern.log,
> lately it just locks up and there's nothing in the logs.

I think the problem is your case is AGP and Mac.  AGP is notoriously unreliable
and the Apple northbridge had additional coherency problems.