KPPP "almost" works

Symtom: My Cendyne serial modem, installed some weeks after I did an installation to hdd using Knoppix (2003-11-14-Beta) "almost" works. It goes tearing out, negociates successfully with my ISP, races to the URL I have selected (in either Konqueror or Mozilla- makes no difference), and pries it open. With Google the target URL, the modem has (once) successfully downloaded a 1.5 meg file. But if I ask the modem to go hither-and-yon (giving the browser a new URL, for example), pretty soon, suddenly the speed drops from pleasingly fast to pita slow, as in roughly a hundred to four hundred bytes at a time, coming...drip....drip...drip. It barely registers on the Kppp graph.
Was ist los ?

My hardware is competent (serves Win2k reliably). The modem which comes from a US warehouse-ful of bankruptcy stock, works fine for my Win2k.
The installation-to-hdd went without incident I said "yes, please" when asked if I'd like network and Samba. Haven't developped them yet.
I don't have any reason to suspect that I have bent the installation. Moreover, I've tried the modem running the cd-os. Same failure.

Diagnosis, please ?
And can someone please suggest where I can read up on the diagnostics available in my installation ? I don't seem able to find 'gkrellm' on the disk. And I'm starting to study 'top'.
TIA,
Chada

PROBLEM SOLVED !
A bright fellow in the Edmonton Linux Users' Group suggested "Flaky UART" as the problem. So I moved the modem to my second machine, ran a new copy of Knoppix 3.3 (Feb), configured the modem, and it "just works"!
Funnily, the UART serves Win2k fine. Still. I'll swap it for curiosity's sake. It's on a Shuttle (!) AV61 board.
I think this newbie has learned a lesson worth sharing: "when at first you don't succeed, try, try again..... NOT !
I'm convinced that troubleshooting a problem is best done looking in the easy places for solutions first, and trying easy fixes first, before looking for obscure solutions and hammering, hammering away.
In an hour, I could have switched to my second (or someone else's) machine.....and saved myself a month of frustration.
chada