2015 iMac 27 i7 cuts out occasionally

I bought a late 2015 iMac 27" i7 on eBay. The seller didn't pack it properly so the screen arrived cracked from one side to the other. I kept it, got a refund, and replaced the screen. I upgraded the 256Gb Apple blade drive to a 1Tb m.2 using an adapter, and it's actually about 40-50% faster than than the Apple drive was. I also added a 2Tb SSD at the same time. It's been working great for over a year.

Recently it seems to cut out - totally stop running and the screen goes black - for about 10 to 20 seconds. Tapping on my touch pad seems to get it back for a bit, but then it cuts out again, sometimes after only 5 to 10 seconds. This is usually while I'm surfing the internet - YouTube... I get tired of it quickly and turn it off. Later, it boots normally and runs fine again. I run iStat menus, and the CPU and GPU temperature is never above 45 - 60 Centigrade, so not too hot.

Any ideas what is going on? Thanks in advance for your help and comments.