Slow/no response

[Ian Carr-de Avelon]

I have a problem that looking up hosts on our system occasionally becomes
slow and then searches start to fail. This even affects local zones. The
problem occures at times of high congestion on the net, which leads me to 
suppose that named is reaching some kind of limit as replies from other
servers fail to arrive, although I emphasise that addresses in the local
zones also fail to resolve. What can I do about this, or what should I look
at to diagnose the problem?
Yours
Ian