I just called in as it happened again (2x in 30 minutes).
At first, the tech could not see the modem. I rebooted and he was able to see and said he saw the problem as the modem switching frequencies. He wasn't sure why - signal or line problem most likely - but has someone coming out in 2 hours, so at least hopefully they'll catch it during the act for once.
What would cause it to switch, and if I am bonding 4 channels, shouldn't it be able to maintain connection better via multiple channels?