There are other Dimension 8400 owners who are experiencing similar issues. See the link below:

http://en.community.dell.com/forums/t/17275466.aspx

There are a number of pages in the discussion so READ THEM ALL. A number of driver updates were attempted and it worked for some and I would recommend updating the chipset, BIOS, and video drivers....but Pages 5 and 6 discuss the SATA Intel Matrix Storage Manager update which seems to fix the issue.

Hope this helps.

Grif