Network freezes around 6 p.m everyday

Hi

I am not sure what is wrong in network settings or some where else
Everyday between 6 to 6:30 p.m. the network freeze and when users shutdown and restart everything works fine

We have windows 2003 business server and all of the clients are XP.

I also see an error in event viewer event ID 1053 and could not connect to the domain controller, I changed the settings in computer configurations in Policies because one of these errors were related to group policies but the problem is not resolved. It is very frustrating for the users, Please help.