We recently started having problems with our Comcast internet where we would lose the connection and the cable modem would just keep cycling through its connection sequence (one LED, two LEDs, etc.) forever. We'd had a similar problem earlier in the week where the Comcast customer service rep was able to communicate with the cable modem but it wouldn't connect. Eventually it was determined that we needed a tech to come out and, perhaps, replace the cable modem so we scheduled an appointment. After hanging up with customer support, I decided to give it one more power-cycle and was surprised to find it connecting. It continued working so we canceled the tech visit but died again this morning.
Multiple restarts didn't work so I called tech support who were not even able to communicate with the modem this time. We were about to schedule another tech visit when I happened to turn the modem over and suddenly it connected! WTF? I told the tech support guy who was still on the phone at that point and indeed he could now communicate with the modem which appeared to be working perfectly. The fact that it started working when I moved it suggested it was a mechanical problem which made me start thinking if there were any mechanical aspects of the cable modem that I might be able to check/fix without a tech. Well..there is that big black cable sticking out of the back of the modem. Duh. Sure enough the cable connector was loose and clearly the problem. Easiest fix ever but probably one that will need to be done again due to the use of the finger-tightened cable connector instead of the old types that required a wrench to tighten.
My theory is that the finger tightened connectors can't be tightened as much as the old wrench-tightened models and, due to the stiffness of the cable unscrew themselves every time you move the cable modem; the connector was getting looser and looser every time we restarted the modem or moved it.
The best long term solution would be to replace the connector with a wrench-tightening type but in the mean time we just put a note on the cable modem to check the cable if you restart it or move it.