I was able to solve my problem with slow speeds. I don't know what causes it, but I have read in a few places that it seems to happen with Routerboards and Arris cable modems.
I solved my problem by replacing the Routerboard 750GL with a new router from Ubiquity (an Edgerouter X). The specs are similar to the 750GL, just with a faster CPU. https://www.ubnt.com/edgemax/edgerouter-x/
I duplicated the configuration, cloned the MAC address to the WAN port, and swapped the devices. Down time was only a couple minutes, and I didn't have to reboot the modem or any other devices. Speeds were immediately back to where they should be.