If you do not have this problem when you are direct connected to a PC (assuming you tried not only data transfers but DLNA as well), the it is not a hardware issue. It sounds as if it is a network issue.
1. How long have you had the unit is service?
2. Is it a Duo or Duo V2? (If a V2, it should state so on the lower right hand corner of the access door).
3. How long have you had this problem?
4. Is the IP provided router a stand alone or a combined modem/router?
Remember - the NAS is not a backup plan.
Backup = Good, No Backup = Bad