I had this problem before when I 1st got my cable modem hooked up. Found out my coaxial cable from the point where it enters my house to my router was too long of a distance. I had to get a Motorolla signal booster. Ever since I installed that I haven't had a problem. I think the tech guy said maximum length the coaxial cable should be is 10-15 feet or so. Mine is about 40 feet on the other side of the basement.

Also make sure your firmware in your router and/or wireless router is up to date.