Debug slow/stalled network connection?

I have a cable internet connection (Motorola “Surfboard” SB5120, Charter cable, SuSe 11.0). This has worked fine for several years, and still works well most of the time. However, every once in a while the connection will slow to a virtual crawl - it will take minutes for something that would ordinarily elicit a sub-second response. Sometimes rebooting the system will fix the problem, but recently that’s also resulted in the connection being unavailable.

I don’t really know where to start looking for the source of this problem. I doubt that it’s anything to do with the particular browser I’m using (Opera 9.51), as I get the same effect when I’m just doing remote logins to other systems.

Could someone point me to a place to start looking?