I think we need more information to help you out on this one.
Is Exchange up and running during the periods when you cannot connect to the server?
Is everyone on the same internal network as the Exchange server is?
Are you able to reach the web interface of the server? Log into OWA?
Are there other connections to the server that work, say A/V updates, opening and saving files. and other such activities?
Can you browse the Internet from the server?
Do the Event Logs from the server or from the PC’s show any reason for the loss of connectivity?
Is your router connected correctly with the incoming line and the line going out to the switches and are the switches properly configured? Someone plugging a loose end into any old port on a switch can wreak havoc on your network.
Do you have more than one DHCP server operational on your network? Don’t just say no because you know that you had setup only one. Someone may have set up a consumer grade router/switch on the network so more PC’s could be hooked up completely unaware of the ramifications of doing so, then connected to the network backwards giving IP addresses to your whole network when the machines are looking for one. Unauthorized routers can be the devil to locate.