HELP! Bizarre FTP Problem!

1. Bizarre FTP/Modem problem: stalls on RPM files

I get consistently good, fast dialup PPP connections from home using
a laptop (kernel 2.0.36) and PCMCIA V90 modem, except for one bizarre
problem.  When I try to download large RPM files (>500kb, say) the
download stalls after a few tens of kilobytes, and refuses to restart.
While the FTP download is stalled, I can still use the PPP connection
to surf the web, telnet etc. normally.

The problem occurs consistently using a variety of FTP clients
(Netscape, lynx, gftp, ncftp), with a variety of hosts (anonymous FTP
servers or various machines), using several different ISP's (including
a few who support V90 and one that only supports up to V34) and over
different phone lines.  I've checked for classic problems such as IRQ
conflicts, and I've also tried using irqtune or tweaking the arguments to
pppd.

A borrowed modem worked fine, so the indications are that the problem
lies with my modem.  However, what is truly bizarre is that only RPM
files are affected, not tarballs, images, etc.  If I gzip the RPM
files before trying the download (which merely serves to juggle the
bits around a bit without changing the file size significantly) then
it proceeds without a problem.  If I use `split' on the RPM file, ftp
still chokes on the first few tens of kilobytes, though it can download
the later parts.

Presumably there is a particular sequence of bits in the RPM files
that is either seen by the modem as an escape sequence (I'm told this
can happen), or that is somehow interfering badly with the data
compression.  What's still more maddening is that I didn't have this
problem a couple of months ago, and it started without my having
conciously interfered with the modem's registers - I have of course
done a bit of this since the troubles started, though I think I've
always reset them correctly.

Anyone seen anything like this before?

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University of Cambridge, Zoology Department, Downing Street, CAMBRIDGE CB2 3EJ

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