Now you see it, now you don't

[jeffchri]

I'm having some issues getting my final renders to, well, render. If I'm in a preview render and hit Final Render, it works under default settings (usually). However, if I turn on Show Shadows for final, it will run through the process (to 100% complete), show the shadows and nice pic for a split second, then revert back to preview quality. No error message, nothing. Sometimes, it's even worse - it will loop endlessly, going from preview to a split second show of final, then back to the beginning (0%).

Ugh.

I'm using a Toshiba M400 laptop, which doesn't have a particularly stellar video card so I wasn't expecting huge performance. However, I wasn't expecting this. Does this sound familiar to anyone? Any ideas how to solve?

Thanks,
Jeff