z77-d3h not booting correctly

[mutilator]

hi been having this issue for a few days when i turn my computer on most of the time it will power up all the fans, hdd etc. and then beep 20 times in quick succession, this is followed by it rebooting and repeating. it will sometimes boot fine straight into windows. when im in windows it could be fine letting type this out for example but other times it will reboot randomly. A friend of mine told me its likely a problem with the mobo and will need to RMA'd but i dont want to send it back if its another problem

i've ran memtest86+ for 2.5 hours with no problems and im on version f18 (latest bios excluding beta)
hardware been fitted for 8 months with no problems till now, no hardware changes.
ill post my spec below,any help would be appreciated thanks.

Summary

Operating System
   Windows 7 Home Premium 64-bit SP1
CPU
   Intel Core i7 3770K @ 3.50GHz   35 °C
   Ivy Bridge 22nm Technology
RAM
   8.00GB Dual-Channel DDR3 @ 798MHz (11-11-11-28)
Motherboard
   Gigabyte Technology Co., Ltd. Z77-D3H (Intel Core i7-3770K CPU @ 3.50GHz)   28 °C
Graphics
   1280MB NVIDIA GeForce GTX 470 (EVGA)   76 °C
Hard Drives
   1863GB Seagate ST2000DM001-9YN164 ATA Device (SATA)   32 °C
   112GB Seagate ST3120827AS ATA Device (SATA)   32 °C
   466GB Seagate ST3500820AS ATA Device (SATA)   31 °C
   1863GB Western Digital WDC WD20EARS-00MVWB0 ATA Device (SATA)   30 °C
Optical Drives
   TSSTcorp DVDWBD SH-B123L ATA Device
Audio
   Auzentech X-Fi Prelude 7.1

CPU
      Intel Core i7 3770K
         Cores   4
         Threads   8
         Name   Intel Core i7 3770K
         Code Name   Ivy Bridge
         Package   Socket 1155 LGA
         Technology   22nm
         Specification   Intel Core i7-3770K CPU @ 3.50GHz
         Family   6
         Extended Family   6
         Model   A
         Extended Model   3A
         Stepping   9
         Revision   E1/L1
         Instructions   MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, Intel 64, NX, VMX, AES, AVX
         Virtualization   Supported, Disabled
         Hyperthreading   Supported, Enabled
         Fan Speed   1205 RPM
         Bus Speed   99.8 MHz
         Stock Core Speed   3500 MHz
         Stock Bus Speed   100 MHz
         Average Temperature   36 °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

Motherboard

Motherboard
   Manufacturer   Gigabyte Technology Co., Ltd.
   Model   Z77-D3H (Intel Core i7-3770K CPU @ 3.50GHz)
   Version   To be filled by O.E.M.
   Chipset Vendor   Intel
   Chipset Model   Ivy Bridge
   Chipset Revision   09
   Southbridge Vendor   Intel
   Southbridge Model   Z77
   Southbridge Revision   04
   System Temperature   28 °C
      BIOS
         Brand   American Megatrends Inc.
         Version   F18
         Date   21/08/2012
      Voltage
         CPU TERMINATION   1.068 V
         VCC3   3.364 V
         +12V   12.096 V
         +VCC   3.400 V
         vAXG   0.900 V
         CPU CORE   0.816 V
         DRAM   1.512 V
         +3.3V   3.384 V

[Fatman]

You did not say what your power supply is. Failing reseatting everything, I would expect that when your PC comes under load it is crapping out on you. That's where I'd be looking first!

[mutilator]

opps..its a antec quattro 850w psu that I've had for just over 5 years. I took it apart and the capacitors have not leaked,  I do have a an old enermax psu but it doesn't have the correct cpu power molex connector so I cant change it to see if its a fault somewhere else in the power supply. I would test it with a multimeter but I don't know if its worth looking at since when I did that list it said the voltages were correct?

When its booted a few days ago i loaded up bioshock infinite and shogan total war 2 and they both played fine for a couple hour on each with no issues.