Is the DSL modem a stand alone modem or does it have a built in router?
If it has a built in router, might need to bridge the modem.
I would check the logs of the 655 router at times when you see loss of internet. Might see something like PPPoE connection down or something to the effect of loss of internet. I presume the router is probably doing this thing and might be detecting loss of connectivity at the modem.