Sipowicz, when the engineer called and before he swapped my modem, my line was spiking erratically. This could be the problem you have. Which modem do you have again?.
I would spend a bit of time explaining to tech support the problems are intermittant and point them to this thread. I think you need an engineer to test your line while your present preferable.
Some of the chaps in here I have received help from my well offer in a pm to help you out.
To help prove my case....
I ran a ping to a well known site something like
www.yahoo.com
in command prompt
ping
www.yahoo.com -t
then... try downloading a BIG file
see what happens to the ping rates my guess is they may say "request timed out"
tell them that might help them understand the problems.