Try to replace the UDP cable. And, if I remember correctly, older Solaris systems have a problem with autonegotiate, in which case both, the switch port and the host network card need to be set to a fixed speed. Link Auto-negotiate is usually default.
In general you do not want to set the speed of the interface manually unless is really necessary.
Check your kstat -p output and make sure your interface is set to auto-negotiate as well as the switch port.