Since then I've not experienced the problem. I don't know what happened, but I haven't seen it again.I just upgraded to Merlin's 378.56_2 firmware. I've had problems with the 2.4 suddenly stopping working once in a blue moon, but since the upgrade it's happened twice in 12 hours. Either the 2.4 channel disappears entirely or it refuses connections.
FWIW, I have different SSIDs for both channels. There is no one on channel 6 on 2.4 except my router. Everybody else seems to be on 1 or 12.