Seen this a few times - especially with the D600. Dell seem to know it's a hardware fault and tend to come out and replace the motherboard (if you're in warranty).
Hope this helps.
Thru...
Happy New Year/Xmas/<insert celebration here>
http://www.acomp.hu/pub/gericom/Note...0Manual%20.zip
Just the book you've been looking for. I was, I found it, and I thought I'd share (as I'd passed thru here on my way to...