Watchdog timeout of realtek 8139

[Ricardo Bermell <rbermell_(_at_)_amutis_(_dot_)_com>]

Has anyone experiencied a problem of 'watchdog timeout'
in his/her network card?

I have a firewall with two identical net cards 
realtek 8139. A pair of months ago one of them 
started to emit this message:

   /bsd: rl1: watchdog timeout

About this error, 'man rl' pages says:

  Diagnostics
  -----------
  rl%d: watchdog timeout: The device has stopped responding 
  to the network, or there is a problem with the network 
  connection (cable). 

Also, connection is broken from time to time (most of cases at 
10:00-12:00 of morning). (The card is the external one of the
firewall)

The obvious conclusion is:
  a) the card is defective
or
  b) the cable is defective 
but physical testing for both possibilities is now impractical 
for me 

Any idea about a more precise diagnostic or software probes?

Thank you

-- Ricardo



P.D. I would expand info (dmesg, etc.) if needed