user accounts lose internet connection

This is an intermittant problem and is happening on a couple of machines.

The internet connection is ADSL, into a netgear router/modem and an eth cable going into the machine. However, I'm not sure that's relevant as the connection seems to stay up and fairly stable all the time. My issue is that although the admin account can use the internet no problem, all the user accounts on the machine lose their internet connection - that is - they can ping the modem/router, but have no gateway to the web (although the admin account keeps it's connection).
The connection will go down for a while and then come back up a while later.

I'm running Feisty ubuntu on these machines.
I have pessulus locking down most things and am confident the machines have not been tampered with.
This is happening on 2 mchines that I know of and is getting more frequent as time goes on.

As these machines have a pretty straightforward install as far as networking goes I am at a bit of a loss as to why unless this is a bug. If it is I'm hoping we can get to the bottom of it at it has become extremely annoying..!

Both machines are identical:
Dell Optiplex 320's
Pentium Duo 3.0
512Mb
I have the same system running on different machines with no probs (that I know of)
feisty is updated and upgraded.

could this be a permissions problem? and if so why is it intermitant?

I'm using Pandora internet radio a lot - could this cause problems? even though its just a browser app?

hopeful,

m