Comcast will be EOLing Docsis 2 modems like the 5100 sometime in late 2013 or into 2014 with the way the speed upgrades are currently going. So if you do have a bad modem, forget about ebay and buy from a known legit source. You took a chance with the used 5100 you bought. Hope you didn't pay too much for it.
These are 2 well known modems and are the current Docsis 3 tech, you'll also be future proofed, depending on how fast you wanna go right right off the bat:
»www.amazon.com/Zoom-DOCSIS-Cable ··· le+modem
»www.amazon.com/Motorola-SurfBoar ··· a+SB6141
There is also a new white version of the 6141, but the specs are the same except for small differences.