Have you ever tried to upgrade the BIOS to 0407 or above?
According to the BIOS ReleaseNotes, BIOS version 0407 or above can fix a problem where system would boot slow with 8GB of memory in the system, and booting to a 64bit OS.(http://downloadmirror.intel.com/17260/eng/DP_0517_ReleaseNotes.pdf)
You have PSU ? how many Watt?
Please look also very carefully your board for capacitors problems- leakage...unusual appearance etc.
Generally, it is probably a BIOS problem. They (Intel) do these things very often- with new BIOS many problems solved-new appears+ old returns.
They are "famous" for the quality of their BIOS... especially the last 3-4 years.
I'm afraid that, the things lately are so bad, that even in the new boards the poor "consumer" is frightened to do BIOS "upgrades"- the chance something to goes wrong is almost 95%. You never know which problems will be solved, which will appear again, which new problems will "show face". Problems you never had before... .
Odd thing is - a quick google search will reveal others running 8GB of RAM in this board...so it can be done. I am thinking I may pull the 2 sticks in there right now (known good) and fire the 2 newer ones in and see if it boots properly. That will ensure the RAM is not the culprit. I have every BIOS issued for this board and may even backtrack thru a few older ones to see if I can get somewhere on an older BIOS....
Regarding the PSU - yes - it's a rock solid Corsair 600 watt...definitely not a power issue...
But there is a problem here somewhere...and I will find it! Or get another board.....:)
Any other advice on what to look into would be appreciated....