I'm still having the same issues, although I'm convinced I've got it narrowed down to 2 distinct problems.

The first is that the temperature gauge simply reads too high. When the engine is running at a cool 178 F, the gauge is just below the terminator between the green and red zones. I'm not sure how these were originally calibrated, but I assume we're only talking about a resistive circuit. I may try to add a resistor to knock it down a little, now that I've confirmed the engine is fine. There may also be a way to re-calibrate the sender resistance. If someone knows anything about that, let me know!

The second issue is that randomly, both the temp and fuel gauges surge randomly up, then fall back down. I'm pretty sure this is a voltage regulator issue. I've tried to clean the contacts, but no effect. A brickboarder was kind enough to offer another voltage regulator for me to try. I'll let you know if that fixes the surging issue.