Unable to browse or ping the local network

Hi,

On a Windows SBS system I am using dual LAN card. One is configured with
real IP another one is configured with private IP. This Windows SBS system
is configured with ISA server 2000 for firewall and proxy. Other computers
are accessing Internet through the Windows SBS system only.

I am facing a problem, some times, mostly in the morning hours any computer
from the LAN as well as Windows SBS system unable to access each other. To
exactly pin point the problem I have found private LAN card unable to
communicate to any other system, not even able to ping.

I have got a work around solution, if I once disable and then enable the
Private LAN card problem is solved. This happen with irregular frequency but
not at day hours.

My question is why this is happening? What exactly the reason? Is there any
permanent solution for this?

If any body can help me to solve the actual problem.

Thanks in advance.

Regards
Swapan