Nice ride! I did a bunch of troubleshooting. I never did get a code reported on the OBDII port. First, I replaced the engine coolant sensor. That didn't have any impact on the gauge reading high temps when downshifting up a hill. So then, I just straight up replaced the thermostat. Cost me $20 for it, and took relatively little effort. I bled the line of air, and took it for a test drive.
Result - rock solid middle of the gauge reading, regardless of RPM's. It would appear the thermostat was the root cause, as it simply wasn't opening up and operating as it should.
