The sad fact of heat issues, is they stress out componets and that in turn starts a slow spiral into failure. If you haven't already, be sure to clean the dust bunnies out and observe that all fans are running good. Then, just leave the system cover open/off and allow it run that way. If after weeks or at least a very prolonged time, it remains stable, return cover. If it then reboots, etc.., then you do have a heat issue. Heat stress will degrade all componets, the weakest link will be telling. I find the psu(power supply) can fail and/or if the overtemp. setting is reached will alarm(if on) and reboot. Also, the cpu could be reaching a tip point and cause a reboot. The mtrbd. may have failing capacitors that in turn cause issues as well, thus inspect any for bulging, leaking, distorted, blown caps. If you can, route all wiring, harness to better the airflow and if possible install another cooling fan. If this is an older system, this is a sign its showing its age. Plus, if you use this system intensively for gaming for other tasks, you can stress componets and heat being an added factor to bring it to its knees. So, your problem while common isn't so easily addressed, everything becomes suspect.

tada -----Willy Happy