The system works unstable

[wishmaster]

Hi!

My 11.0-STABLE system had been working stable for a three last years, but became unstable a week ago. The system just freezes about once/2 days (at night) and no responsive via network, nothing in the console.
Before the first freezing I had made updates of applications in the base system and jails.
Options VIMAGE and Linux compat. are enabled.

The first thought was bad sectors on the HDD, but  dd if=/dev/ada0 of=/dev/null bs=128k rejected this one.

How can I debug this situation?

Thank,
Vit