Intermittent system shut down & restart

I have a system that i built myself and ive occasionally upgraded it and added things to it:

M/Board - Biostar TPower I45
Processor- Intel E8400
RAM - OCZ Reaper 1155 mhz 4GB
ASUS - 4890 Gfx Card
Samsung- 1TB HDD
Seagate - 500 GB HDD

Those are the main ingredients of the system.

I have recently bought a CM Storm Sniper case with 900 watt Ultimate UCP power supply.

Ever since i moved everything into this new case and changed my PSU for the CM 900 w
i have noticed my system restarts itself randomly. I have had it running for almost 2 weeks
now and it has restarted 4 times. I have no idea what could cause this, i know that motherboards
can arc and cause a system to restart but thats the only thing i know can cause this. The PSU
is an ultimate circuit protection model and since i dont know what that is or how it works i thought
that it may be viewed as a possible culprit.

My system is overclocked from 3ghz to 3.8ghz and my gfx card is slightly overclocked with the ATI
CCC software and i have no heat issues on this set up.

If anyone could help me solve the restarting problem please be my guest because i could do with
the help and i really dont want to send the case and psu back because ive sent one back already
and stripping and rebuilding becomes a pain, besides, im running out of cable ties.

I will do that, the reason i havent done it already is because i wasnt sure the
PSU could account for that symptom.