go into your modem's configuration and check your signal to noise ratio
my guess would be that you have a weak signal at your modem
make sure you use no splitters, amplifiers, etc. between the line coming into the house, and your modem. It should (except for one main split, as soon as the cable comes into the house) be a straight line to your modem. Also, make sure the main splitter is as close to where the cable comes into the house as possible. The weaker the signal is by the time you split it, the more it degrades it. It's exponential, if you will.
If the signal's still weak, call your cable company.
And if at all possible, you should position the modem as close to where the cable comes into the house as possible. I know i have my network setup where the signal only has to travel through about 3 feet of cable to the modem after it comes into the house, and i've never lost my connection once since i moved here in august.