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Surface Pro 3 Dock Audio Device keeps disappearing


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Had the Dock for a few months with no problems. One day, listening to Spotify through the dock/external speakers, the internal speakers will start to play, then after a period of time, play through the external speakers looked up to the Audio Out port on the dock. Outside of the dock, the SP3 plays through the same external speakers with no problems using the headphone port.
Bringing up the Sound/Playback tab, I can see the docking station audio device ( set as default), disappear, the Realtek HD Audio device ( internal speakers) come on, then the Dock audio device will reappear as the default device, and play again through the external speakers.

After a call to MS, they determined it was a faulty dock, and three weeks later, I received a replacement, only to find the same behavior. I've uninstalled/reinstalled the drivers: same behavior. I can use the headphone port on the SP3, and after changing device to "Headphones", the SP3 plays back through the external speakers without incident (while in the dock). The rest of the dock functions are normal. I have tried other external speakers with the same results. Any ideas?