after upgrading from linux-image-2.6.15-23-powerpc 2.6.15-23.35 to linux-image-2.6.15-23-powerpc 2.6.15-23.39 the freezes went away, but at the same time the drm driver fails to initialize; therefore I've opened Bug #46578 instead -- linux-image-2.6.15-23-powerpc freezes on pbook https://launchpad.net/bugs/45884