Have 8gb ram, 4 gb usable while few days ago had 7,50gb usable. Checked everything, any suggestions?

Hello, I have any issue with my memory. As tittle says I have 8gb installed and windows show it up, but only 3,97 gb usable with remapping on, while I turn it off its 3,47gb. Thing is while I played bf4 once I got blue-screen and after crash I let windows fix fails. After that i got 7,5gb usable ram. few days ago game crashed again with 0x0000007E blue-screen, and after i restarted it my usable ram was tuned down to 3,97 gb once again.

I updated OS and BIOS and it didn't help.

I did thing in ms-config but it didn't help. I also checked memory with windows memory diagnostic, and it didn't find any failures.
Also checked ram if its not broken or something. Every brick is working and is connected properly.

I run >
Windows 7 Eternity 2009 64-bit
Intel Core i5 750 @ 2.67GHz
8,00GB ram 1600mhz 1,25 voltage (8,00GB Dual-Channel DDR3 @ 669MHz (9-9-9-24)
ASUSTeK Computer INC. P7H55-M (LGA1156)
1024MB ATI AMD Radeon HD 5800 Series (XFX Pine Group)

Any suggestions?

spec 2, dont know if it helps just some info from indle pc.
Motherboard
Manufacturer ASUSTeK Computer INC.
Model P7H55-M (LGA1156)
Chipset Vendor Intel
Chipset Model DMI Host Bridge
Chipset Revision 11
Southbridge Vendor Intel
Southbridge Model H55
Southbridge Revision 06
System Temperature 29 °C
BIOS
Brand American Megatrends Inc.
Version 1302
Date 2011-04-25
Voltage
CPU CORE 0,960 V
MEMORY CONTROLLER 1,776 V
+3.3V 3,360 V
+5V 5,040 V
+12V 11,765 V
VIN6 0,288 V



RAM
Memory slots
Total memory slots 4
Used memory slots 4
Free memory slots 0
Memory
Type DDR3
Size 8192 MBytes
Channels # Dual
DRAM Frequency 665,1 MHz
CAS# Latency (CL) 9 clocks
RAS# to CAS# Delay (tRCD) 9 clocks
RAS# Precharge (tRP) 9 clocks
Cycle Time (tRAS) 24 clocks
Command Rate (CR) 2T
Physical Memory
Memory Usage 74 %
Total Physical 3,97 GB
Available Physical 1,03 GB
Total Virtual 7,93 GB
Available Virtual 3,75 GB
SPD
Number Of SPD Modules 4

Graphics
Monitor
Name BenQ G2412HD on AMD Radeon HD 5800 Series
Current Resolution 1920x1080 pixels
Work Resolution 1920x1030 pixels
State Enabled, Primary, Output devices support
Monitor Width 1920
Monitor Height 1080
Monitor BPP 32 bits per pixel
Monitor Frequency 60 Hz
Device \\.\DISPLAY1\Monitor0
ATI AMD Radeon HD 5800 Series
Manufacturer ATI
Model AMD Radeon HD 5800 Series
GPU Cypress
Device ID 1002-6898
Subvendor XFX Pine Group (1682)
Current Performance Level Level 0
Voltage 1,175 V
Technology 40 nm
Die Size 334 mm²
Release Date Sep 23, 2009
DirectX Support 11.0
DirectX Shader Model 5.0
OpenGL Support 4.1
GPU Clock 865,0 MHz
Temperature 45 °C
Core Voltage 0.950 V
Bios Core Clock 865,00
Bios Mem Clock 1300,00
Driver version 13.152.1.8000
BIOS Version 113-587XZNE12
ROPs 32
Shaders 1600 unified
Memory Type GDDR5
Memory 1024 MB
Bus Width 64x4 (256 bit)
Pixel Fillrate 27,7 GPixels/s
Bandwidth 166,4 GB/s
Max Power Draw 188 Watts


CPU
Intel Core i5 750
Cores 4
Threads 4
Name Intel Core i5 750
Code Name Lynnfield
Package Socket 1156 LGA
Technology 45nm
Specification Intel Core i5 CPU 750 @ 2.67GHz
Family 6
Extended Family 6
Model E
Extended Model 1E
Stepping 5
Revision B1
Instructions MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, Intel 64, NX, VMX
Virtualization Supported, Disabled
Hyperthreading Supported, Disabled
Fan Speed 1188 RPM
Bus Speed 133,8 MHz
Rated Bus Speed 2408,4 MHz
Stock Core Speed 2666 MHz
Stock Bus Speed 133 MHz
Average Temperature 40 °C
Caches
L1 Data Cache Size 4 x 32 KBytes
L1 Instructions Cache Size 4 x 32 KBytes
L2 Unified Cache Size 4 x 256 KBytes
L3 Unified Cache Size 8192 KBytes