T7400, beep code for memory problem, motherboard?

Basically once it simply didn't power on. Or rather it immediately shut down, right after i pushed the POWER button. Took out power cable, replugged, messed around the power button a lil, and there we go, - it didnt power off, but.. instead BEEP CODE for memory problem started going off, don't remember the numbers exactly, but i checked with some documents on the dell web site, and it stands for memory problem. The unit has 2 memory expansion cards, filled by 4gb x 16 slots = 64gb.

I thought one of the modules is the problem, so i disassembled it, and tried those 1 by 1, but.. all of em were working, i mean it would boot up to bios just fine, so i thought maybe one of the extension cards itself broke, tried both of those on the Precision 690 Unit - both of em are working fine.

Anyway, i started trying to boot up T7400 with memory raisers inserted directly into the motherboard, works with 1, 2,3,4,5,6,7 and then, when i insetr the 8th the beep code goes off again. Rechecked the memory module itself - works,  tried to start up the unit with 4gb x 6 slots, - beep code again, removed 2 (to 4gb x 4 slots - boots up like:

Dell SAS 6 Host bus adapter BIOS

blahblahblah

detects my hard drive (1,5TB raid system)

Dell Inc. MPT boots successfully installed!

No device found

AHCI BIOS not installed  

time to time  it also adds somth like "Error in the DIMM 1 or 2 module (ran with only 2 raisers installed)"

F1 to continue, F2 to run the Setup untility,

F5 to run onboard diagnostics.

diagnostics says that everything is fine

If you push F1 , windows begins to load and the blue screen of death pops up.

 Thought that maybe there's not enough power(1kW supply), for 2 CPUs nVidia quadro FX5600 n stuff, so i switched to some simple video adapter, from the Precision 690 unit, disconnected everything except hdd's and memory, and  the beep code returned %)

So i think that.. i dunno.. motherboard?