I have used RedHat and gnutella/browsing over a modem, and never had problems, other than a reasonable slowdown from the increased traffic. Maybe it is the upload traffic that is hurting you. Do you get problems from heavy upload traffic on another program, such as an FTP upload?

I also found a problem sometimes with K56Flex and V.90 procols. They are supposed to negotiate correctly, but I have gotten better performance manually picking the correct match.

You can also use minicom to talk to the modem, and use AT commands to report connection errors.

Just some ideas. (and of course, be sur
to check ppp and system logs.)