cable signal is too low

once you are able to get on line, type:
htt://192.168.100.1
it will bring you to the cable information page. click on signal, for a cable modem to work you need:
downlink: -8 to +8
uplink: 48 or below
C/I: 30 or above

remove any unused splitter in your cable system (from the entry point, where the cable enters the house, up to your cable modem). make sure the cable goes to your cable modem has minimum loss (at most 3db lower from the entry point), etc.
call you cable company if your signal is very low, assuming you are using at least SB5100 (less quality cable modem may higher signal level to operate properly). they should check your cable connections, etc.
at last if above did not solve the problem completely go get a signal booster (allow passive reverse link) to put at the entry point of the cable.