https://bugs.freedesktop.org/show_bug.cgi?id=95017
--- Comment #15 from erhard_f@mailbox.org --- Played around a bit with xorg and kernel options on my G5 and worked out that I can drive the Radeon 9650 at AGP x8 if I disable DRI completely via Option "NoAccel" "true" in /etc/xorg.conf.
I had no GPU-lockups since then and xorg is much more responsive despite lacking DRI hardware acceleration (radeon.agpmode=-1 with DRI enabled).