What fixed mine? I have an Apple Airport Extreme dual-band AC router. I read feedbacks from others about ensuring the 5ghz connection name was seen as an option to log on to my network. After clicking the box that said to broadcast the 5ghz connection name, and logging in to the 5ghz signal...I'm locked in at a steady 866.5mbps.
I also have an Airport AC. So, just to clarify what you're saying, you checked this box to assign a unique name to the 5GHz network:
And then connected the SP3 specifically to that named network?
I will have to give this a try when I get home and report back.