Connection drops

I manage a network with over 60 small remote sites with 4 to 5 mixed Windows machines. Some are running 2000 others are running XP.

Occasionally some of these PCs become unreachable.
I can ping them from any PC on my network but I am unable to BROWSE to them and all shared resources become unavailable.
I have checked the Task Manager and processor is at 0% during these events, netork traffic is minimal.

– It appears that only NetBIOS traffic is being blocked. There is no Firewall running on the PCs.
– Event logs reveil nothing

This happens with both operating systems.

The only thing I can do to recover from this is reboot.

Any suggestions would be greatly appreciated