I was running OS X Lion without any problems until I installed Parallels 7. Now when both are running and I'm working in Lion, frequently my cursor hangs and the only way I can resume normal operation is to bring up the Force Quit dialog. I don't have to actually force the app I'm running to quit, just bringing up the Force Quit releases the cursor and I can close the dialog and the cursor responds as it should. This is something that Parallels is doing, because it doesn't happen when Paralles is shut down.
