Cannot stream video from Diskstation via Wifi

I'm experiencing problems streaming video from my Diskstation DS212j to my Sony Xperia Z tablet via my Vodafone VDSL HG659 modem.

Video will stream ok for around 10 minutes, then continually freeze with buffering problems.

I am confused as to why this is happening as the streaming from DS to tablet is via the home wifi and does not relate to the underlying speed of the internet. We are in a small apartment, so the Vodafone wifi router is reasonably close to the tablet.

So any feedback will be appreciated.

Is it a crap router?

Is it the aging Sony Xperia Z tablet (I doubt it as it can handle streaming from internet ok most of the time)?

Is it a configuration issue with the DS (I use the DS file android app on the tablet to stream the video file on the DS)?

cheers