Hi there, I think I might have an idea.
A year or so ago I started getting the exact same problems with my computer. I use a router, and I tried a direct connection from the broadband line to the computer and it didn't help and so that led me to think it was a problem with my modem.
I'm with telewest, or Virgin Media as it's now known, so when I phoned up the tech support line, I was tole to open up my web browser and in the address line, enter in an ip code, something like 190.40.683.27 and it took me to my modem diagnostics page, where it turned out my download frequency was different from what it should have been.
It was easily corrected and it just recently did it again, same problem, same solution.
That might be your problem.