Some folks have had issues with UPSs in the past.  When troubleshooting, always disconnect from the battery backup.

Hope that's the fix! Wish I would of seen it listed in your specs. Usually people dont even list their psu let alone a UPS.

I never use a UPS.. had one thet kicked the bucket decades ago and never bothered again. I guess if you have power brown outs of spikes often it would be a good idea.

It's a big battery and it will wear out eventually from recharging often.

Possibly cant handle the quick power draw of your system.

80c.. yeah not good. Might want to set a more aggressive fan profile for the card to keep those temps down in the 60s if possible. Something thet kicks up the fans sooner to keep the card from getting toasty then kicking in at 100% or so when it's too late.

Low power can cause all sorts of stuff to go off.. electronics like the motherboard needs certain power to run stable.. chipset cpu,memory, gpu and drives.. would seem feasible.

Yeah I'd try to replace the battery on it if possible or just grab a bigger model that meets the systems demands if your having power issues in your house.

Prob during the summertime when everyone is running their air conditioning. Winter close so ya got some time to find a replacement I bet.

I'd still say grab a Kill-A-Watt locally at a hardware store or amazon and install that on the outlet and connect your pc to that to monitor the voltage and wattage used at peak pc usage.. it can monitor your voltage at the outlet alone so you can see if the volts hold 220 stable. Much lower or higher can point to an issue at your fuse box or service. If so.. you can test it alone on multipal outlets around the house to find issues.

Sajin
Glad to see the ups was the issue. Must have missed that detail.

ehhh not so fast. got through 3 whole matches of Fortnite without a restart then I joined a 4th and got a restart. dammit. guess I'm gonna have to swap GPUs after all.

Still using a new psu as well?

Alright.. wish ya luck! Let us know.how the next card is.

I didn't want to post this too soon but it looks like I fixed it. The PSU was faulty.
I did switch GPUs. I put my 1660ti in my old PC and I put my old PC's R7 250 (which I didn't know was 250..speccy/device manager just say 'r7 200 series' but I looked at it the card itself and it says AXR7 250) in the new PC. Neither system crashed.
Then I put them back and it seemed like reseating the 1660ti fixed it but eventually I started getting restarts again. So I put my old PC's not-so-old Insignia 450W in the new PC and after thorough usage I haven't had any restarts or freezes. Thank you everyone for your help.

 
Random restarts are usually caused by the PSU. So I'm not surprised that was probably it.