What I did:
1. Updated the stock kernel to 2.6.20-16 from 2.6.20-15 that came on the CD., rebooted with no problems
2. Installed build-essential etc.
3. installed linux-source-2.6.20 , unpacked it in /usr/src , made a symlink
4. make-kpgk clean ; make-kpkg --append-to-version=-mychanges --initrd kernel-image modules-image
5. 3 hours compile time later... resulted in a 24MB-large package in /usr/src
6. dpkg -i linux....deb
point 4 is your problem
It should be:
4. cp /boot/config-`uname -r` .config && make oldconfig && make xconfig (or "make menuconfig" if in text terminal - <ctrl><alt><F1>)
4.1. make-kpkg clean
4.2. make-kpkg --append-to-version=-mychanges --initrd kernel-image modules-image
You've had ommited most importand step - configuration.