Forgot the memory controller is in the CPU, did you update your BIOS or check to see if there is an update that addresses this? I have the same issue with memory sticks that have an EPP timing set, and can't use that, as well, my motherboard doesn't support it.
Im downloading the manual to see if I can find anything... -.-
EDIT:
After looking through your manual, out of the box, it is only certified to run on Corsair CM2X1023-8500C5 or GEIL GB24GB8500C5QC for the 1066 frequency, and it's limited to 1GB per stick sadly... See if there is a BIOS update that allows more per slot. I remember in the old days if you ran to much ram, it would literally force a downclock for some reason, that may be what you are seeing now....