Random reboots

[Sbix]

First of all I am not 100% sure if that's the correct place to put this topic, so please excuse me if I made a mistake. Thanks!
Now for the issue- My computer started randomly restarting itself. At first it was just the monitor going to sleep after 15 minutes of inactivity and then sometimes being unable to wake up unless I use the power button to restart it. (Neither keyboard or mouse were able to "wake it") Not a big issue as the monitor is very old, but has no other problem really and it wasn't happening often. Later, however, I had to restart the whole computer for it "wake up". I say wake up, because I have turned it off, as well as hibernation, since they were giving me problems in the past. (The warranty covered it back then, seems like the motherboard was faulty. Even so I had those options turned off ever since and its only the monitor that lights off.) And now the computer started rebooting at times. Most of the times were in-game, but once it was while talking on Discord with nothing else on. After a reboot I quickly check the temps- everything is around 20-40 degrees. For that I am using Speccy. Today I even had a hard crash which resulted in the computer loading up an older safer version. Didn't see anything removed, so not sure how old of a version it rolled back to.

I have checked all my drivers and they are all up to date. The dust has been cleaned off recently and from I can see/feel all the fans are spinning normally.
Specs: Win. 7 Ultimate 64-bit SP1; i5-4460 CPU @ 3.20Ghz; 8GB RAM; GTX 960. The OS is installed on an SSD everything else is on an HDD. Apart from the rebooting I haven't noticed anything else weird.
What can I do to troubleshoot, or find the issue? I'd change the HDD/SSD, but if its the power supply?! And vice versa. Same for all other parts really.
Again, sorry if that's the wrong place for this topic and if my question is stupid/badly asked.
Thank you for your time!