TCP/IP Gateway

I have a network of 10 pc's running on windows 98, they
login on a windows NT4 Server. In addition I have an
internet router and a kilostream link to another office.
Both of these links are setup as gateways on the network.
All of the network with the exception of a newly installed
PC work perfectly well. The new pc did work for half a day
and then would not ping the kilostream link. I changed the
IP address and it then worked for a short time again but
then stopped again. It sees all of the network, including
the fileserver and the internet router but not the
kilostream!!! I'm totally baffled. Can anyone assist?

Regards

Stuart