The version on the latest board in question is 2.9.23
The previous problem APs/bridges have 2.9.19, 2.9.29, and 2.9.23, respectively.
The SBC is a routerboard 532A and the BIOS has not been updated.
We are using a UPS as well as a line conditioner and remote power controller.
Also, we haven't tried a different kind of card. We have only used Ubiquiti SR5 cards thus far.