Hi. I'm facing some problems with Dual Channel Memories on GA-EP45-UD3R

When I put memories on Slot 1 and 2 (Single Channel), the PC boots and works fine.

When I put any stick on the 3rd or 4th slot, the PC doesn't boot, keep beeping and restart forever or freezes after 5 min.

This started to happen after I cleaned the parts and changed my case.

Can you guys help me? This is MB problem or can be the CPU?

1 Stick on Slot 1/2 - OK
2 Sticks on Slots 1 and 2 - OK

1 Stick on Slot 3 - Doesnt boot, many beeps and restart
1 Stick on Slot 4 - Doesnt boot, many beeps and restart
Both Sticks on Slot 3 and 4 - Doesnt boot, many beeps and restart

My PC:
C2D E4300 1,8 @3,0GHZ
GA-EP45-UD3R
2 x 2GB GSKILL PI BLACK 800MHZ DDR2
Xigmatek Dark Knight S1283V
PSU BFG Tech GS 550W