OK. A ways back on this thread when I originally had this problem, someone said "I have a bunch of these docking stations in service and have had no problems - return it and get a replacement, it's defective." (actually, I think it's the person posting above me saying for the second time "take it back")
I should have listened.
After my post above about it being the external USB ethernet adapter - I factory reset the SP3 (fourth time in three days) and reinstalled everything without ever connecting the USB ethernet adapter - and the problem came back!
Last Friday I bought another full SP3 setup - SP3, type cover, and docking station. It was being configured for another of my users but today, just on spec, I took the other dock and connected it to the same ethernet cable and port, dropped in my SP3 - and bingo, a solid connection.
Contacted a Microsoft Store rep on chat - they said "oh, only 30 days warranty on accessories, we can't do anything for you" but then suggested I phone technical support.
Tech support set me up with a walk in appointment at my local Microsoft Store (luckily there are two in the city in which I live) and my dock was exchanged for a replacement.
Returned to the office, connected the new dock, and immediately got another solid connection.
If you're struggling with the same issues of occasionally but rarely getting a network connection, you might want to consider whether there could be a few bad apples in the bunch of docks out there (and mine was from the original first release in August 14 but the replacement dock is from the same lot, 1433).
With any luck, this will be my last post on this thread!
I get the dhcp issues a lot like you described but on wifi for me
