ipw2100; poor throughput from linux-2.6.20.11

Hello,

I have an IBM ThinkPad R40, with Slackware 11.0.0 and kernel 2.6.20.4.
It detects and loads in ipw2100 to operate the built-in wireless card,
which runs fine. I can scp a large file (e.g. a kernel tar, 42MB) from
a local machine wired to the router/wireless access point at a steady
600KB/s.

I've been trying out later kernels, such as 2.6.21.{1,3}. The wireless
interface also works on these, but with irregular and reduced throughput
on the same scp. It starts off quite small (<200KB/s), then gets worse,
often stalling, and averaging out at about 10KB/s.

I've experimented with various intermediate kernels, and found that
2.6.20.10 is fine, but 2.6.20.11 has the problem (though perhaps not
quite as bad: 350KB/s down to 20KB/s).

What can I do about this? (I can happily stick with 2.6.20.10 in the
meantime.) Are there any settings I could poke around with? Where can
I look to get diagnostic info? Is there a better place to discuss this?

I can't try the wired interface to see if it's just a wireless problem
because it's broken. I have a PCMCIA wired interface, but that stopped
working around 2.6.18 - I'd been upgrading to see if that problem had
already been fixed (and I'll probably start a separate thread about it
at some point).

Thanks,

Steven