Question : Network connections and/or speeds plummet daily, usually late afternoon

I have recurring network speed issues very similar to that described in this post (http://www.experts-exchange.com/Operating_Systems/SBS_Small_Business_Server/Q_22078159.html).  Basically, various machines in my network lose connectivity with the server or experience drastic reductions in speed on a daily basis, usually between 4:00PM and 6:00PM.  This is not an internet bandwidth issue; the main problem is with connections to the server or printing on the network and can be fixed with a reboot of the affected machine.  All the following machines are regulary affected, although never all at once and rarely on the same day:

Notebooks 1, 2, 3, 4:  Wireless through Buffalo access point
Desktops: 1, 2, 3:  Ethernet (10/100 cards)
Notebook 5 Ethernet (gigabit card)

All the machines are running XP Pro, and the server is running MS Server 2003, SP1, with ten licenses.  It is used only as a fileserver & domain controller (my terminology may be off here as far as the domain controller is concerned; all I'm trying to say is that this machine is where I keep all the usernames/passwords, permissions, etc.).  I have a Netgear gigabit router, and that connects to a Netgear gigabit switch and to the Buffalo wireless access point.

This problem affects different machines, but never too many at a time and always at the end of the day.  I'm stumped-   Help???

Thanks,

jdj

Answer : Network connections and/or speeds plummet daily, usually late afternoon

Was that an average or a peak?

Could there be something not network related going on?  A server maxing out on CPU or disk IO?