I've been running Ubuntu 8.10 since pre-release, and it's been very stable for me, on multiple machines.
As your description of the problem indicates, the very essence of the problem appears to be related to software you're applying to the system after a successful installation.
If you're updating the system with Ubuntu-provided updates I'm really not sure what the problem could be... however, you mention updating "debs" on your system, which sounds like they may be unofficial/unsupported.
What .debs are you installing? If the system stops working reliably after their installation then I would take a closer look at what they're doing/the services they provide. Do you really need them on your system? Can the needed software not be found within Ubuntu's official repositories?
Hope this helps.
(Your English is very good, by the way