What is your S/N Ratio....Forward Power ... and Return Power while connected and online?
(If anyone asks about grounding at the wall plug, forget it. The Motorola S/B's only have a 2 prong plug. There is no physical grounding for the in-line wallwart.)
Spit out all the facts of the install if you can describe it all :) How many A/O's in the place? Any other digital devices on the house side of the D-Mark? Is there a House Amp installed? If so where? How many splitters after D-Mark?
Your devil is in the details of the install. Sorry to hear that you've gone through so many modems and have to keep going back at it.
RadioMan