Internet Connectivity Problem

[wave]

For some reason, my internet cuts out about every 20 seconds and then starts back up again a few seconds later. It seems to cut out more frequently while I have multiple programs open which require the internet. I have a Linksys router model BEFSR81 and a PCX1100 Toshiba cable modem. The lights on my cable modem are stable so I'm thinking the problem is my router. I called Comcast and they said everything appeared to be working from where they were. I also reset both my router and modem and I tried switching ethernet cables. Any suggestions on how to fix this problem?

 

[HoopaJoop]

Have you checked your machine for spyware/adware and viruses?