I am out of town right now, (with my car dropped off before I left) so I can't confirm any of this myself yet, but my dealership claims a solution has been found.
This is my third trip to the dealer for the temp gauge needle problem. Once again they confirmed it themselves that it was happening, but this time they say I'm not the only person whose been in for this issue. In fact, they now have an official Subaru publication on the issue, which says it's due to loose wires in the engine bay.
True or not, at any rate they say they applied the fix in the publication, and now claim that on subsequent test drives they now no longer see the problem. They seemed a lot more confident this time than in the previous trips, where they replaced thermostats or did coolant flushes, and kind of shrugged and said "Hopefully that's it...." (which it wasn't)
The fact that they claim there's an official Subaru bulletin on this now is good news. Hopefully it's the reason some of us are having this problem.
I get home on the 27th, I'll drive around myself and report back!