I have an SB5100 that has begun to intermittently stop working. This has been happening once in a while for about 8 months but has begun to be more frequent.
When this happens, the power and receive lights stay on and the send light flashes but the online and pc activity lights are both off.
The modem was being kept in a closet with limited ventilation and I found the sides of the unit to be a bit warm so I was thinking that it might be getting too hot so I would unplug it for a while and give it a break. About half the time this does the trick and the modem comes back with all lights and connection restored but the other half the time I have to wait until the connection comes back on it's own. I have also relocated it out of the closet but this seems to not improve the operation of the modem as it has still cut out on me in it's new cooler location.