Slow network connection

I have one NT 4 computer on the network which was set up a long time ago and has suddenly decided to slow down on the network. I’ve changed the network card, reinstalled network services, upgraded from SP5 to SP6a and connected it directly to the main switch. I can ping the loopback and it’s own IP with no errors but there is allways one or two errors pinging gateway from said computer.Any ideas as user cannot download emails?
I’m off to change network cable in desperation.
Peter