We had a similar problem last week, turned out to be local fault which took 2 days to fix. What operating system are you running? If possible, next time it goes bad, try running a tracert/ trace route to a site like
www.bbc.co.uk or
www.google.co.uk and post the results.
Easiest way in Windows XP is to open start menu and select run and then type
cmd
then in the dos box that opens type
tracert www.bbc.co.uk>c:/trace.txt
this will run a trace to
www.bbc.co.uk and then output the result to a file in your C drive called trace. You can either attach that here or copy and paste the results into a post.