I would check the basics first ( sounds as if you've already been through it all thought ) , such as ping several servers, google.com , testmy.net , ect.. trace route them and see the latency differences. Once you get a trace route, find the highest latency and trace it out. You can use different means to ping ( again , I'm sure your aware of ) ICMP / TCP. Only reason I mention this is I did not see you post any results along these lines.