That happens quite regularly when you try to bring it back from a minimized window.

One thing is the amount of lights you have, I know it's a personal thing but since P6 you can get away with a lot less lights than in previous versions, maybe think about using less lights more effectively.

Also sometimes it's better to render the image and shadows seperately and then composite it together in photoshop or similar afterwards, this method saves an awful lot of 'puter resource!

Injustice will be avenged.
Cofiwch Dryweryn.