Lock-up Affecting Our Entire LAN?

I work as an administrator for a small business (~30pc and a Windows 2003 R2 server ).Recently, we started to have problem with our lan, the network start to kinda lock-up randomly. For example, browsing folders is fine then you switch to one and the explorer window just do nothing (with a loading cursor) and after 4 second everything refresh and work again, then in word or any other program (sometime not even related to network files/folder) you press the save button and it do the same, do nothing for 4-5 seconds then refresh and work again.I'm positive that no process on the server take up enough CPU time to do that, it's not a per-pc problem since everybody is affected when the problem start and when I reboot the server it work like a charm for another day then the problem may restart the next day.Nothing show up in the event viewer of both PCs and the server and we can't find a correlation between the problem and high CPU/LAN usage on the server (at worse CPU is used at 10% and LAN at 2%).Our RAID controller and our SAS HDs are working flawlessly, we're in the process to change our infrastructure anyway but since I can refer to people that may know their stuff way more than myself I though Id ask here.We first though that our switches may be in cause but then why rebooting the server would be correcting the problem ?