Music pausing/stuttering

I just installed Plex on my Mac (OS 10.6.8 and iTunes 11.4) and the app on my iPhone 4s (running iOS 9.1).

I’m trying to work out the reason for the stuttering - is it my iPhone or the Bluetooth connection to the speaker (a NEET B-Box)? Does the iPhone need to be nearer the Mac with the installed server? Or is it a Bluetooth issue?

Thanks in advance.

Happens to me on Windows using the web client to play a music playlist. Happened on the previous PMS and the one I updated to today does it also.