PA1s keep locking up and needing power cycled

I have about a dozen PA1s throughout my network.  They keep randomly locking up or dropping off the network and need power cycled.  Some are on the latest firmware (8.7.5.75), some are on older firmware.  Some are running off POE and others are running off AC (as an attempt to fix possible POE issues).  They're plugged into a mixture of switch models (Unifi and Zyxel).  They'll drop off at random intervals.  The switch will still show a link light but the device is unpingable, doesn't answer at the web gui, and doesn't play anything when making a page.  All lights are on the device.  

Any suggestions on what I can do to diagnose what's going on?

Thanks