I had a mysterious broadband problem a few months ago - weird things like mail mysteriously refusing to log in, as well as slow line speed- called BT, and after they'd threatened me with £180 fee if they dind't find a fault, a linesman came out and found the wire broken, but making intermittent contact, at the box on the post. Some of the protocols retry until they succeed, others don't, hence the different behaviours.

Other possibilities are a compromised computer (a "something unauthorised" eating into your bandwidth), contention (try at different times of day/ night), problems with your firewall, contention, a failing ethernet port......

You can eleiminate some of these- try with another computer, just bang a fresh Linux into an old machine and fire it up. That will eliminate worms etc., and also the computer's hardware, and since it's just for test you can do without a software firewall safely. Reset the router - turn off for an hour or so so you're sure you get a new lease at the other end.

After that, it's just the usual half- truths peddled by ISPs, if you can find a better 'ole...