[plug] [network newbie] diagnosis sick network

[Sol]

Hello!

I'm having an odd problem with a small network. Every so often - every so many days - the connection between the server and the rest 
of the network ceases. I can't ping the server from the client, nor the clients from the server which is a major bummer considering 
two clients a thin clients. Yet the clients can ping each other. Whilst I suspect a hardware fault - I usually get things working 
again by run route and *wobbling* the cable until I get a response and then reset the networking daemon for good measure - but I'm not 
absolutely certain. It could be a dodgy ethernet card?

What I was wondering was if there are any diagnosis tools that I could use to help narrow down the cause? 

Thankyou,

sol