Blouboy,
Have about the same hardware you have(PowerBook G4, 17inch, 1,67Mhz, 1,5 GB memory, ATI Mobility Radeon 9700 graphic card, Mac OSX 10.4.11, SU hardware acceleration and fast feedback on) and of course SU 7 installed.
I have tried all kind of different things, using the right mousebutton, but nothing abnormal.
I cann't repeat your screenflashing/blinking at all.
I am sorry I cann't help you. The only thing I can say is that it isn't a general Mac problem.
I hope you find a solution. If so please keep us informed, others might encounter the same behaviour.