Weird diagonal pattern on window when moving to 2nd monitor (Parallels5)

[rwbaker]

I'm running Parallels5 on an iMac with a Dell 21" screen hooked up as well. When using Crystal or Coherence, if I drag one of the windows to the Dell monitor, I get a strange diagonal display of the window. Screenshot attached.

Dell 21" Widescreen flipped portrait 2007WFP; 1050x1680 @ 60Hz
iMac (late Fall 2009) 27"; 2560x1440
Parallels5; 512mb video ram total; 256mb video ram for virtual

Any ideas how to fix this?