PC Pitstop
If the only online gaming you play is thru battle.net, it may be the only time you seriously test your connection enough to realize there is a problem.
Run the traceroute utility. Instead of pinging PC Pitstop's webpage, put in
www.battle.net or whatever the correct address is. Then you can see if a particular router is giving you problems. If you have a lot of packetloss or high ping on your first or second hop, than it's your connection has the problem.
If everything looks good, look around and run some of the other tests. Maybe it's just a setting you need to change.
Or, maybe your dsl is just garbage. Wouldn't be the first time.
Broadband Reports.com is also a great site, but I notice they've started charging for some of their tools. You can still find tons of info concerning broadband in general, and your ISP in particular.