Drive seems to "sleep" (by 2 users)

My DSM-G600 is connected to a switch (DLINK 10-100 switch) and this is connected to a router (DLINK G604T). All addresses in my home network are static. I succed in mapping the NAS drive but I have randomly occurring problems transferring files from this unit (even trying to read some multemedia file) to local PC or other network nodes. It seem sometime that, while trasferring, drive stop suddendly and start again after many seconds (30 or more). What can cause a problem like this one?

Thanks.