I know that command Z is for undo, Ive been at this a few years, not that I'm a pro, but I've never had this problem. When I do command Z in this one motion project the viewer goes black, all my media is still in the project pane, I can select all and see all the media outlined in the viewer but no visual of the media in the viewer. I end up reverting to last saved and all is well until next time I try and undo. I've opened up other projects and don't have this problem. Also my F8 has quit working I just get the dreaded Bonk.
iMac 24" 4gb ram 130 gb internal Hd - 5 TB external HD