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. The patches i applied lately were [PATCH] radeon: allow write_reloc with unaccounted buffers to cope with Mesa bug https://patchwork.kernel.org/patch/4663071/ and drm-radeon-fix-TOPDOWN-handling-for-bo_create-v3.patch https://www.mail-archive.com/dri-devel@lists.freedesktop.org/msg132931.html My guess would be patch number two.