I have be use a Motorola sb5120 -lrb- mostly happily -rrb- for a couple of year and plan on upgrade we network start with a new DOCSIS 3.0 cable modem , the Motorola sb6121 . today I just try install my second modem from Amazon after the first give I the cycle-on , cycle-off problem some of the reviewer have mention . sadly the second one be the same , at least for I . I have to conclude either there be 1 -rrb- a batch of defective modem out there , or 2 -rrb- it really be not compatible with Comcast cable in Portland , or . there be no point in order a 3rd . Comcast do not give I any assistance - as per usual , they first suggestion be I buy the ` wrong ' modem -lrb- tho they do not exactly say that , and the website give it all high mark -rrb- , and they attitude be one where I imagine the service tech greet I with `` hello , this be Comcast , how can I help you ? let see we record indicate that you first name be `` Moron '' correct ? yes , I think so ... '' -rrb- -lrb- to be fair , he really do not say that and be rather quite pleasant , he just could not offer any meaningful help . he do even call back to check if I work out my problem -rrb- so apart from my Comcast minus rant , at this point there be no way I can do better than 1 star for the sb6121 though I wish I could . on the flip side , `` thank you Amazon '' for you awesome return policy . this one will be back home with you in a few day .