The 115Kbps is no doubt the reported "port speed", i.e. your serial port's
maximum speed (UART). You should be getting anywhere from 40-48Kbps on a
56Kbps modem (some lucky people get more). The troughs in speed are probably
drops in line quality between your connection and the UKLinux lines (though
I rarely had trouble with them). One thing you can do is have BT test your
line quality, and then prompt them to increase your line's "gain". That
Another thing to do (though I'm personally not sure how ;-) is try and turn
off compression on the modem's software. Having an ISA modem doesn't help
either, as the extra overhead can make things slower than if you were using
a direct external modem (my preference) or in fact a PCI (but its hard not
to find one that's exclusively WINMODEM these days).
Anyway, hope some of that helps. Good luck!