I'm having the same problem with a DH55C. Like you I've tried single dimms, unplugging HDDs (not the OS drive), rearranging dimms in the memory banks. I think this started with the June BIOS update for my board.
Did the problem start after a BIOS update?
Are there any Intel folks that can shed light on this matter? Unrelated boards with similar problems may suggest a BIOS problem.
Thank you and sorry Hank. I'm not high jacking I'm just really frustrated with this problem because my system is a dream otherwise!
I still have a problem with a system not booting on cold mornings and believe it must be a power supply issue. After several minutes I have pressed the power button, intending a soft off after 5 seconds, but the log-on screen has appeared immediately. The BIOS does not matter. Perhaps it is the graphics card but the problem didn't appear in another location with another power supply and different peripherals. I don't want to do a reinstall so I may tolerate it for a while.