Self-compiled kernel problems

[Mark <spamavoider@gmail.com>]

I am encountering inexplicable problems when using a custom-compiled
kernel under Sarge.  My greatest complaint is extremely slow dialup
PPP connections.  (About 800 B/s compared to 4.5-5.0 kB/s using the
prepackaged kernel.)

Versions:
gcc: 3.3.4-13
binutils: 2.15-5
working kernel: 2.4.27-1-686
kernel source: 2.4.27-6

I tried multiple kernel configurations and after finally trying the
configuration in /boot/config-2.4.27-1-686 and encountering exactly
the same problems I decided something really strange was happening. 
Can anyone shed any light on this problem or recommend anything else I
can do to fix this?

Mark