Since you replaced the sending unit I would tend to believe the temp gauge. The heads will run hotter than the intake, that's why I like to take the temp from the intake near the thermostat. One thing I have done to my engine, is drill a small hole (1/16th) in the thermostat to keep a good supply of hot water moving up to the thermostat. I've found this keeps the engine temps more stable instead of the big swings I used to see. Difference for me is, the engine being all aluminum, it was sucking so much heat out of the cylinders, I felt the engine was getting to hot before the thermostat would open. Once I put the bypass hole in, everything became normal.