I have a Mac Pro Mid 2010 with two ATI Radion 5770's driving 6 Dell U2412M's. Problem is, when even just one of these displays is rotated to portrait, the entire system grinds to a halt. CPU usage spikes to about 85%, I have 12gb of RAM installed and when this happens, about 512mb is free. And graphics performance is all but completely dead. At first I thought this may be because I am driving six monitors at once. But this is illogical be cause for one, it only just started occurring since I upgraded to 10.8(which as a power user, I absolutely hate). And two because this also happens with just one monitor hooked up. Any ideas?
This is the sort of thing a well documented bug would let an engineer fix the code to avoid crippling a powerful Mac with a video change.