Domain Server problem

I have a problem that name resolution does not work, eg. 'pinging' google.com gives error 'unknown host google.com'.

I can cure this by using command:-

/etc/init.d/networking restart

but I have to do this ~every day. The network in general (serving web pages etc.) works ok (as far as I can see). Does anyone else have this problem or any idea what's up? I am running Ubuntu 6.06 on host55.

Thanks