I have three Surface Pro 3 Docks that were having this issue. After replacing the display-port cables that were supplied with the Dell monitors with some Star Tech cables the issue disappeared.
Thanks. Does the startech cable seat further into the SP3 dock port and the monitor port? I wonder if all of the Dell mDP to DP cables are suspect now. What specific Dell monitors are you using? I want to look up the Dell mDP to DP cable part number.
If have a U3014 panel and experience sort of the same issues, except that it happens to me also when the machine is alive and I am basically just working on it.
It starts flickering for a few seconds (sometimes a minute) and then everything settles down.
I read somwhere that the Surface dock does not properly work with mDP2DP cables, so I replaced it with a mDP-only cable, and to my surprise everything worked just fine.... Until I pulled out the SP3 from the dock. At that point the monitor appears dead (buttons not working, indicators dead) and does not recover until I pull the mDP cable from the dock itself. Then (and only then) the monitor comes back to life.
So there is definately something dodgy going on. If I connect the mDP2DP cable directly to my SP3 (not using the DP-out from the dock, that is) I have not yet encountered any issues.
Also, I have had a Surface Pro 2 (and 1) with the "old" dock, and I never had any problems using the same monitor.
With the Dell supplied mDP-2-DP cable I was unable to retain a picture for more than an hour, and then it would start flickering for about a minute or so.
With the Dell supplied mDP-2-mDP cable I had no flickering issues but as soon as I removed the Surface from the dock the monitor would get "stuck". Unable to recover unless unpluging the mDP cable from the dock.
In a final attempt to get this problem fixed on my Surface Pro 3 I ordered a different mDP-2-DP cable (1 metre) and a mDP-2-mDP cable (also 1 metre). The first resulted in flickering only. There was never a stable image, so WORSE.
The mDP-2-mDP cable however worked flawlessly and so fat the monitor hasn't got stuck. So the brand of cable might in fact be solution to the problem. The cable I am now using is a Delock cable that will set you back around (9 euros).
It looks like my problems are finally solved.
I have the exact same set up as the OP, JohnMc10 except I've got Surface Pro 3 i5 and I'm having the exact same issue.
I shouldn't have to spend $50 on new cables to get my $550 monitor to work as designed 😞