it would be good to see a topology of your network to help troubleshooting.
is the uplink between your switches? if so, you could have a loop in your network,
do all the activity lights light up on your switches whan you plug in the uplink?
Verify all the cables between the switches and make sure you don't have a loop,
virtual server hosts with multiple nics, unmanaged switches and hubs can also cause this.
You can also look in the logs to see if there are spanning tree errors, or
look at spanning tree status to see if there are ports that have a 'blocked' state, that would indicate spanning tree has shut down a port,
if the uplink is to the telephone network... is this an analog line?
you say standalone network, so I assume it's not an ethernet cable...