Any computer experts out there want to try diagnosing this for me...

My setup is cable modem / wireless router. Monday morning I was getting an IP address but not connecting. Called cable company, they had me take the router out of the loop and it still wasn't working, so they said it must be my Norton Antivirus. That didn't explain why my roommate wasn't getting anything through the router either, but I went with it. Spent an hour dealing with the Norton help person, and got it working on my system without the router, but still wasn't working when I added the router back in. Reinstalled the router Monday night, and it worked. All was fine until this morning, when it suddenly died on me, completely. Again I took the router out of the system. Still didn't work. I took the power cord from the cable modem and put it back in, and this got it working without the router. For about an hour. Then it died again. Did the same thing. Worked for 10 minutes, then it died again. Tried putting the router back into the system. Didn't work. Tried it again just the cable modem, unplugged everything for a while first. E-mail worked, but the web browser didn't, and Norton froze up every time I tried to send outgoing mail. Seemed all screwed up. So I unplugged cable modem again and then plugged it back in. And it's working again, for now. But I have no idea what the problem is.

Seems like it can't be the router, because troubles even without the router. And can't be Norton, because troubles even when I disable Norton, and because my roommate wasn't able to get a connection through the router either. So it seems like some sort of weird intermittent problem with the cable modem. But they told me it wasn't them. Am I not thinking of something obvious? Any ideas what it could be, or is it definitely the cable company? Or maybe the physical cable modem? Or is this not what would happen if either of those were the problem and it must be my computer?

Thanks. :)