U3415W, Surface Pro 4, Surface Dock, mDP to DP Speakers detected intermittently

We have a Dell U3415W connected to a Surface Dock (the new one that looks like a brick) via mDP.

It seems that after a few days (sometimes a few hours) the Surface stops detecting that there are speakers connected via the DP port.

If you reboot the surface, they come back and work for a while again but eventually they'll stop working and audio only comes out of the surface speakers.

Seems to be related perhaps to the computer going to sleep and waking back up, being disconnected and re-connected but sometimes it disconnects while its still powered on and connected to the dock.

I've tried everything suggested here. 

Any further troubleshooting steps would be great.

Thanks all,

Ryan