I downloaded Lion to my new iMac on day two. Since then I've experienced something I never saw with Snow Leopard or on my MBP. Three times now, twice when trying to watch a news video on Safari, and once in iCal, the entire system has locked up. Only the cursor moves. That's it. Buttons and function keys are dead. Nothing will launch, nothing will even happen. Battery power is 90+%. I finally do a power button shut down, then a reboot, and all is well. Until the next time. Help?