Dell Optiplex 755 won't stay turned off

[curmudgeon]

I have a strange problem with a recently acquired Optiplex 755.

With the machine powered off, every eleven minutes and six seconds (plus or minus two seconds), the machine "turns on" for about four seconds (the lights come on, and the hard and optical drives start spinning up) before returning to the "off" state. This is very undesirable behavior (aside from the fact that it puts unnecessary stress on the hard drive).

I have gone through all of the BIOS settings to make sure I don't have anything set there that would cause this (most notably, the "AC power restoration" is set to "off").

Posting this on the Dell forums got me a reply of "this is controlled by the operating system, and you need to install a supported operating system (windoze xp or vista)."

I am hoping that the people here are smarter than that. Any ideas on the cause (and ways to eliminate this)?

Thanks.

[keenblade]

It seems the machine does not power off, but goes into a kind of sleep mode. Interesting.
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[Jaglover]

Does Dell have any BIOS upgrades for this machine? Clearing CMOS may be worth a try.
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[curmudgeon]