asterisk users - Oct 2004 - "zt_get_index: nullok is not asserted" could led to freeze?

Hello all,
we have been random freezes in two asterisk systems more or less once a 
day last mounth.
In both system we are using a T100P card and a channel bank for local 
extensions (fxs) and h323 or sip (we have tried both with same results) 
voip gateways for FXO connectivity.
We used to have some SIP local clients using SER but those were taken 
out trying to fix the problem.
When the system freezes it keeps responding pings, and the channel bank 
doesn't loose T1 syncronism (the T1 led keeps green) but nothing else 
responds (no ssh connection, no console response).
Channel bank gives dialtone but no call can be made. In calls in 
progress  the voice start do be more and more discontinous and then you 
hear only silence.

Once we reset the machine we don't find any error or warning in the logs 
(asterisk, kernel, messages, etc).
We are running sysstat in one of the systems and the only clear sympthom 
we see is the cpu usage raises to
100% just before we have to reset the system.

We are also capturing the console output to a file and we have noticed 
that (sometimes) there is a message like this in the console few lines 
before the end (not allways and not only, but...):

zt_get_index: Unable to get index, and nullok is not asserted

Does anybody knows if this message could be related to a system freeze

Thanks very much for any advice (and I'sorry for my poor english :( )
Luis