I understand that your system is rebooting randomly and you will like to know the cause.
In regards to the problem that you are experiencing, the reboots can be caused by several factors and it will be very hard to determine the cause without your full system configuration and troubleshooting performed so far.
I recommend you to confirm that your processor, power supply and RAM memory are fully compatible. Test the system on a basic configuration and try the motherboard out of the chassis. If you have the option, try swapping parts and testing the system with no RAM memory to check if you receive beep tones.