I have seen this behavior now, on the iMac using the ati driver on a Mach 64. I suspect it's due to the PPC patches not being applied to the 4.1 source (they need to be reworked). fbdev driver works - although slow.....
Stew Benedict -- MandrakeSoft OH/TN, USA http://perso.mandrakesoft.com/~sbenedict/ PPC FAQ: http://www.linux-mandrake.com/en/demos/PPC/FAQ/