Optiplex 755 - Strange Problems

Greetings!

I have a Dell Optiplex that was given to me in known working condition.  It has sat for about 6 months, unplugged, just sitting on the floor.

A couple of days ago I decided that I'd like to set it up as a little storage server.  When booting though, I would not get any video.  I reset the the BIOS on the motherboard, and then got video, but only for a short time.

The video shows the Dell Splash screen and BIOS Version.  Then goes to saying AHCI Bios not loaded (no devices found).  It then very quickly shows a "Press F1 to continue or F2 to get into setup", and then simply goes black.

If I press F2, I get into the CMOS setup, but only for about a second.  Then the screen goes black.  I've quickly hit the down cursor key when in setup, and the focus changes and continues downward, but the screen still goes black after about a second or so.

I have thus far:

• Disconnected all drives from both power and data cables
• Disconnected the ethernet cable
• Disconnected the multi-card reader
• For Video I've tried
  • The supplied ATI dual VGA card
  • Another PCIe card
  • Onboard Video
  • A VERY old PCI video card
• Tried various RAM in each of the slots
• Made sure that only keyboard and mouse were plugged into USB, nothing else.

I'm STUMPED!!

This seems to be a very nicely build little unit, but for the life of me, I cannot figure out what the heck it going on!

It is running what looks to be a pretty old BIOS (A10), but since it was working with that BIOS before, it's hard to believe that suddenly something is incompatible... especially with everything unplugged from the motherboard.

I'd be happy to try a BIOS upgrade, but I'm not sure how I would even go about that in the current situation.

ANY help or ideas would be appreciated,

Thanks,

Jie