Network often not responding

[Akhmad Sakirun]

Hello,

I have squid cache server using:
MB Tyan Tiger K8W/S2875
Dual Opteron 246
4 GB DDR,
connected to cisco catalyst.
Freebsd AMD64 6.1-Stable

sometimes network down and up again on uncertain condition
i've tried change network card  and disable onboadr gigabit ethernet,
work good for a days then it happen again.
Then i try to move another port on catalyst switch, it's same.

When network not responding connecting using cross-cable not work too.
it will up again 5 minutes later or more.

I can't find any relevan error on /var/log/message except something
like this:
Aug  9 15:09:16 cache kernel: xl0: transmission error: 90
Aug  9 15:09:16 cache kernel: xl0: tx underrun, increasing tx start threshold to 120 bytes

anyone experienced something like this ?