windows 10 – PC crashes – Looking for guidance on troubleshooting

Not too sure where I can start with troubleshooting an issue I’m having – open to any/all suggestions.

PC is a couple of years old and up until a few weeks ago, it has been running without issue.

Recently the machine will die and instantly reboot – as if the power has been switched off then on again. No OS crash/dumps/logs. It only occurs when playing games – but I cannot re-create the issue at will. For example, I could play Apex Legends for 2 hours straight and not have one crash. Then I could fire up another game and it happens in the first 3 minutes.

It doesn’t seem to be linked to any specific game. The machine can sit idle/browsing the internet and has not crashed.

So my thoughts are something GFX or PSU related – there’s no BSOD or crash screen to make me think it’s an OS failure. OS and GFX drivers are up to date – the only recent change is a new, bigger monitor but the problems started several weeks after that was setup.

Any suggestions on how to troubleshoot this other than swapping out parts?

Specs
Win 10 64bit
Intel Core i5 6600
32GB RAM
ASUS z170-E
NVIDIA GeForce GTX 1070