named stop answering

Need help!

We are an ISP running named 8.2.6 on solaris 8. We have been running
this version for one year without any problem.
One week ago, we began to have some problems. Sometimes, named stop
answering without any error in the
namedlog and whithout using any cpu. To remedy to this, we have to do a
"kill -9" and restart the process.
What should be the problem and what would be the solution?

Any hints would be realy appreciated.

Thanks!