We have a 100Mbit company network with approx. 50 workstations (mainly XP-W8, mainly DHCP), several servers (miscellaneous), some other devices (VOIP adapters etc.) and two Zyxel ES-1552 switches. There is a domain (AD/Windows Server 2012) running.
There are really annoying freezes in network communication, lasting from several seconds to more than one minute, concerning many but not all(!) workstations (no matter DHCP/static IP). These freezes occur usually in the morning when the network traffic is stronger.
For example, there are three XP workstations (A, B, C) in the same location. When the freeze occures, A and B completely stop networking (no internet/intranet, Windows explorer does not respond due to mapped network drives, A cannot ping B or anything else (timeout)) and it can last for more than a minute. At the same time, workstation C does not face any problems. Afterwards, the communication is restored and all three workstation communicate without any problem.
I'm rather a programmer, not a professional network administrator. I tried to reset switches, view logs, I even installed Microsoft Network Monitor but no help. There were several major changes in our network infrastructure in the last three months (a new server, new workstations, domain) but I cannot connect the problems with a particular event.
What are the suggested steps to diagnose this problem (without putting the network down)? Many thanks.