I have few questions and a couple suggestions:
- What your raspberry pi are doing exactly ?
- What software are you running on them ?
- Are they working at max CPU usage ? If yes, can you cook an egg on them ?
Monitor your CPU usage, RAM usage and temperatures to exclude anything related to your use (that because you seem so sure that the problem is not your PSU). If your RPi get hot, they may overheat, then slow down so much that it looks dead... and will need to cool down before accepting to restart. If remains powered on, even with restart if your services are restarting automatically, they will not cool down quickly. Multiple reboot by changing PSU, cable and things like this may be just enough to get them to cool down and sudently restart normally.
I had this exact same experience with my RPI and it took me a while to troubleshoot only because I had one of those ridiculous crap of heat sinks sold with some case or RPi kit. I ended up going overkill cooling by designing this : https://www.thingiverse.com/thing:2598644
. This overkill system without any fan keep my RPi 3 at 50-55 degree C running at 100% 24/7 (motion software with 2 USB HD camera running as CCTV). Since I run with this no problem any more, even with non official RPi PSU. Temperature are going down to 30-35 with 40mm fan on top the heatsink.
So maybe double check your CPU is not overloading really slowly, pushing into its limit after a year, even more if your RPi are running 24/7 without interruption at all (you mentioned UPS, so I guess you need no interruption).