While I don't have a precise answer for you, I once had a notebook which behaved in a similar manner. In my case, the motherboard was failing, rather than an attached device, causing increasing instability. I was lucky, because I caught on fairly early and had good backups.
If possible, try removing the drive and putting it in another computer. Perhaps running the live CD again and attempting backups.
Just happened upon this thread
. Don't know if it will be helpful, but... Read the last few in particular.