Hi @MrTaylor,
I also faced the same situation multiple times in my network. When I reboot the AP, everything starts working perfectly for some period, but after that, issues arise. I test the cable as well, then I reset my AP and upgrade the firmware. From that moment, everything is working completely fine, but I don't know what works here: upgrading firmware or resetting the AP.
Try different troubleshooting step by step, and let me know as well what works for you !!