PC reboots once after fresh boot + only uses half the RAM

[cisko116]

Good day!

Recently I've been setting up my old PC for my girlfriend to use, but I keep running into two problems:

1. After a fresh boot (occasionally after a reboot) the PC reboots itself once while the 'Loading operating system.." message appears for a split moment. Boots fine after said reboot.
2. PC states that 8GB RAM is installed but 4GB is usable.

What is interesting is that the random reboot only happens when computer boots without sounding the single POST beep. When it boots with the POST beep, it boots without rebooting itself.

Things I have already done
:- Reinstalled Windows
- Installed all updates
- Installed latest drivers
- Updated BIOS / undid any custom settings
- Ran MemTest, 1 pass without errors

Setup:
MOBO - GA-P55A-UD3
CPU  - Intel i5 750
GPU - Radeon 6870HD (1GB)
RAM - 2x KVR1333D3N9K2/8G (2x4GB DDR3-1333MHz)
     - (are in dual channel slots, running single channel?)
HDD - WD2500AAKX
     - (Swapped SATA0>SATA1 port on MOBO because I broke the SATA0 port during setup - no settings adjusted)
PSU - Huntkey HK600-53AP (600W)
OS   - Windows 8.1 Pro

Hit me up with anything useful - I've tried everything I could think of.
Thank you so much for the help and kindest regards.

 

[Lutfij]

I'd look at the PSU as the weakest link here. See if you can source a reliably built unit. Which two slots are you populating the rams on your board? Did you verify your installer isn't corrupt? Speaking of which, which version of BIOS are you on and what is the revision for your motherboard?