[Help!] Intermittent reboots on COF8 / UniKernel

I upgraded to COF8 about a month ago but have only been experiencing these reboots recently. Not sure what has changed. I tried different versions of UniKernel but same same. I'm afraid it might be a hardware overheating problem, but the phone doesn't even feel hot when it reboots. Very random. I can be using the phone for an hour without a hitch, then run VSCOCam for a minute trying to edit a photo and it will reboot.

I've tried doing a factory reset in TWRP then Odin-flash COF8 and kernel again, but still the same problem.

Any idea?
Thanks.