Hello,
Maybe it is something in userland, but I would be interested to know
what happens if you switched to a kernel from Debian Lenny e.g.:
http://packages.debian.org/lenny/linux-image-2.6.26-2-xen-686
http://packages.debian.org/lenny/linux-modules-2.6.26-2-xen-686
or an upstream kernel.org.one.
Cheers,
Andy