Basically it is a motherboard issue. Updating the BIOS, replacing the heat sinks and both fans didn't make any difference. It took me 7 months and multiple home visits and hours on the phone before they finally fixed it. If the BIOS update didn't work (I still have R166551 and R215335 on my desktop from their remote logins) INSIST they change the motherboard while it is still under warranty. Tell them it is a known problem and that it has been an ongoing issue in the Dell forum for nearly a year. I cut and pasted the link to the forum for the tech I was on chat with...that seemed to work.
Also, I noticed I get much better service through the chat interface than on the phone...not sure why. Good luck!