I have a Network Attached Storage placed in my room attached to the wifi router where my DSL modem is connected. I have another wifi router setup as AP that is connected to the main router via UTP cable.

I have put all my MP3s in the NAS. When I access these MP3s via my PC in my room, they play smoothly as if they're stored in the PC's hard disk. But if I play these MP3s using another computer that is connected to the AP via wifi, the songs skip every few moments.

The signal strength between the other computer and the AP is 4 bars. What seems to be the problem?