8Gb Memory DDR3, recognized 6Gb usable 4Gb

Hi,

I just got a brand new mobo with a set of two kingston ddr3 stick of 4Gb each. I set up everything in my pc and when I install Windows7 64b, strange things start happening with the RAM. First let me tell you the details:

MoBo - Gibabyte GA-A75M-D2H (Checked that it can handle 16Gb).
Processor: AMD Athlon II X4 641 Quad-Core Processor 2.80 GHz
RAM - Kingston 1333 8Gb set 2x4Gb 1,5V
Video - Geforce GTX 560Ti
Windows 7 Ultimate 64 bits

As I said, once Windows loads, it says Installed Memory (RAM) 6,00GB (4,00GB usable).

Checking with CPU-Z, it tells me te following:

Northbridge AMD K12 Bridge rev. 00
Southbridge AMD A55/A60M rev. 11
Graphic Interface PCI-Express
PCI-E Link Width x16
PCI-E Max Link Width x16
Memory Type DDR3
Memory Size 8192 MBytes
Memory Frequency 666.7 MHz (3:20)
CAS# latency (CL) 9.0
RAS# to CAS# delay (tRCD) 9
RAS# Precharge (tRP) 9
Cycle Time (tRAS) 24
Bank Cycle Time (tRC) 33

Memory SPD
-------------------------------------------------------------------------

DIMM # 1
SMBus address 0x50
Memory type DDR3
Module format UDIMM
Manufacturer (ID) Kingston (7F98000000000000)
Size 4096 MBytes
Max bandwidth PC3-10700 (667 MHz)
Part number 99U5471-020.A00LF
Serial number 7114D37A
Manufacturing date Week 15/Year 12
Number of banks 8
Nominal Voltage 1.50 Volts
EPP no
XMP no
JEDEC timings table CL-tRCD-tRP-tRAS-tRC @ frequency
JEDEC #1 6.0-6-6-17-23 @ 457 MHz
JEDEC #2 7.0-7-7-20-27 @ 533 MHz
JEDEC #3 8.0-8-8-22-30 @ 609 MHz
JEDEC #4 9.0-9-9-25-34 @ 685 MHz

DIMM # 2
SMBus address 0x51
Memory type DDR3
Module format UDIMM
Manufacturer (ID) Kingston (7F98000000000000)
Size 4096 MBytes
Max bandwidth PC3-10700 (667 MHz)
Part number 99U5471-020.A00LF
Serial number 4713AE39
Manufacturing date Week 15/Year 12
Number of banks 8
Nominal Voltage 1.50 Volts
EPP no
XMP no
JEDEC timings table CL-tRCD-tRP-tRAS-tRC @ frequency
JEDEC #1 6.0-6-6-17-23 @ 457 MHz
JEDEC #2 7.0-7-7-20-27 @ 533 MHz
JEDEC #3 8.0-8-8-22-30 @ 609 MHz
JEDEC #4 9.0-9-9-25-34 @ 685 MHz

Also, in the gigabyte utility program, detects the 8Gb.
Now, when I start the pc, the count only goes up to 4Gb, and in the BIOS only appear 4Gb.
I've been reading some posts of people who had the same problem, and first I took out both Dimms, cleaned them up and reseated them, nothing happened, same results.

I've tried both of them connecting them one, starting the pc, and then the other one. Both work well, but something weird happens whe using the second slot. Let me explain (sorry about my english, but I am from Spain). When I connect one dimm in the first slot and I start the pc, everything is ok (same when I do it with the second dimm and the same slot), but if I insert only one dimm in the second slot, the pc seems to start, but the screen remains black.

I thought the second slot might be broken and I cleaned it up with isopropil alcohol, but same result.

What bothers me is that if this second slot is broken, why CPU-Z reads the 8Gb perfectly?, even identifying the serial number of each ram dimms. Same thing happens when I check them with gigabyte's utility.

Should I try to change something in the BIOS?. Please help, I could send the Mobo to DRMA, but I will have no pc for a long time.

Thanks