That's a tough one to troubleshoot.
- When gaming CPU temperatures go through the roof. If it goes beyond the set point in the BIOS the motherboard will shutdown. Not reboot. So I doubt you are hiting this temp limit since you are rebooting and not shutting down.
- Next is a video card issue. This will cause screen tearing and fragmenting. Basically your game frigs up for a few seconds then windows may blue or black screen then may reboot. You are getting a straight reboot without issues so it's not a video card malfunction.
- Next is the reset button. Maybe the wiring or button is shorting causing a reboot. This only occurs in games so I doubt you have an issue with that.
- 2 things I am suspecting. Under the full load of games, your power supply may be providing too much power beyond it's ratings and it's built in protection is causing a reboot. Over power protection or something like that. Video cards will draw more power under load in games.
- Last it could be a software issue. So windows, that particular game, or some drivers. Just something quirky cause the reboot. Even some wierd malware or something.
My strategy would be to try a higher powered, or new power supply. See if the problem persists. If so then just take an hour and reinstall windows. It'll take ages to troubleshoot so just reinstall windows to save time. If after that the problem continues I'd suspect a bad motherboard.
Is it only in 1 certain game, or in all games. If it's 1 particular game it could be software related.