Well, I'm not sure what did it, but now my dash gauge operating temperature is back to normal. It's also displaying more "normal" behavior--IE, before the engine was getting to the low 1/3 on the gauge that was about it. Now it gets up to the 1/2 point, and then I see a decent drop as the thermostat opens, and then it comes back up.
I changed the lower intake manifold gasket, replaced the thermostat, flushed the radiator (had a minor but still worrysome amount of brown sludge), and refilled. I don't know which of those might have done the trick, but I'm happy with the outcome!