Reboots while gaming are mostly caused by two issues, either your system overheats or your PSU is dying.
For monitoring temps, there are several programs. I prefer Speccy,
Do some gaming and keep the eye on temps. If the reboot isn't caused by overheating then it's time to get a new PSU.
Here i suggest Seasonic in the 500W range,
E.g S12II-520 or M12II-520 EVO.