Hi out there!

I'm a bloody Rookie in Autocad 2008 and I've got following problem:

Sometimes when rendering a view of a Autocad 2008 file, the screen turns black. If I zoom out and render again, it works, but i'm not able to render Details. Restarting the file without saving changes solves the problem sometimes, but my work's of course gone!
Any ideas, what this could be?

Thanx a lot