While a drive mapping ("net use") or other network connection will generate a little traffic between the client and the server from time, as either the client or server passes an authentication token or refresh, it shouldn't be a significant amount. Can you give me an idea of how much data is being transferred when you see these spikes?
If you have SAM, you can also open the realtime process explorer and/or the realtime service explorer to see which programs are using a large amount of CPU and/or RAM - a large file transfer *should* also have a program associated with it.
Even without SAM, you can do the same thing by opening Task Manager and seeing what's running at that time (make sure you check the "show processes from all users" option).
My gut says to check clients for virus, malware, trojans, etc. If that much data is being transferred, SOMETHING is happening on the box.