There could be several reasons why this is happening. Your modem
could be trying to retrain, in which case you'll have to get out the
manual and force it to connect at a lower speed. You could have line
noise problems. Most modems have a terminal function that will test
line noise. If your signal to noise ratio is lower than about 40 db,
that would be your problem.
John 'Zaphoid' Krutke
This is a very smart answer. I'm impressed :)
Not many people understand how latency and packet loss affects things
like this, and how high speeds can work against you, and not for you!
Good job :)
-Larry