random internet lag spikes

I use comcast cable and for no visible reason and in no pater my internet will work for 5 seconds then go off for 5 then go on for 10 then go off for 3 (or any other set of random numbers usually shorter than 15 seconds). The problem effects all 3 of the computers in our house. A friend told me to try pulling the power cord on my modem and router to fix the problem. When I do this the problem is fixed for like 5 minutes then starts again. We have a netgear wireless router (not sure the exact model) and we have the modem that comcast installed. Another friend said it was that comcast uses cheap modems and that's the problem. Before I go and buy a modem I don't need what do you guys think the problem is?