#1 guess 15 minutes is just about right to over heat the processor if the heatsink is not makeing a cood contact and with enought therimal compond (since you just built this unit)
#2 guess A bad dimm is a posibility. assuming more than 1 dimm, you may want to reduce to 1 dimm and test it to see if the failure goes away. They start adding or swapping in the other dimms till the problem comes back.
#3 what is the system doing when it reboots? Try booting into buis set-up and just let it sit and see if it reboots when not in the OS.
If it only reboots while in the OS, update the BIOS & Firmware, then check to make sure you have the newest drivers installed. allof which should be on the support page for this board.
I checked and confirm Crucial* CT2KIT25664BA1339 is not among our tested memory lists; see the following URL:
Crucial does validate it, though:
Still, if you could, if it's withing your possibility, try with any of our tested memory.