PC randomly shutting down/restarting

Bullets so it's easier to read for you guys

• It happens completely at random - PC could be idle on desktop and it could happen
• Started happening about a month ago
• PC is a year old
• There is no blue screen
• Toggling off 'shut down on system error' (or whatever it is) didn't work
• When it happens, sometimes the PC gets stuck on the motherboard logo
• Went from happening a few times a week to at least twice daily
• Cleaning out the dust (though there was very little..) from the PC caused it to stop happening for a day or so, maybe a coincidence
• Temps are fine

Parts:

• i7 7700K
• MSI Z270 SLI Plus
• Aorus GTX 1080 Ti
• Corsair CX650
• 16GB Corsair RAM (I actually forget the exact name/model sorry)
• An old Seagate 1TB HDD and a 500GB Samsung SSD

My intuition is telling me it's the PSU, however I don't have another PSU to test. Other sites say it's possibly the motherboard. I don't want to commit to buying something if it doesn't solve the problem so any advice appreciated.

Btw if you're wondering why such a cheap PSU with a high end rig - it's because I was planning to reuse my old PSU from my older rig, but the cables weren't long enough, and I had no way to extend them, so I traded it against a new CX650.