So much of the time Internet problems result in vendors
pointing fingers at each other, while you try to figure out
what's really going on. DNSstuff.com is packed full of tools
that can help you decide where the blame should be
targeted.  It's not real intuitive, but with a little playing
around and researching your results on the web, you
can get a feel if your results indicate problems. I'm 
surprised how often I find bad DNS servers. It's one of
the major factors in Internet performance for the user.