Lossing network connectivity

[Francisco Reyes]

Have a new machine that I installed 5.3 release.
Has been working for a week. Yesterday it started to loose network 
connectivity.

ifconfig -u shows the interface as active, but ping to any other machine 
in the subnet fails.

Looked at /var/log and did not see any errors or messages to hint what the 
problem may be.

Switched the port in the switch where the cable was connected and tried a 
different cable from another machine that is working.

A reboot seemed to fix it momentarily.

Booting windows just to test connectivity and windows had connectivity 
with no problems.

At one point got "no media" no matter which cable/port I used, but then 
it seemed to see the network all by itself again.
There was one message on the screen this time
SK0 watchdog timeout.

appears and after that connectivity is restored... at least for now..

Any clues?