Guys, do I have a temperature problem?

Gentlemen: I'm searching for a solution to a vexing temperature problem in my very stock 1974 Duster. My car has the 225 engine, stock renewed head, EGR system disconnected, all in very good condition. When I start out, the coolant temperature takes about five minutes to climb up to about 210 degrees, then regulates back down to 195. If I run the car for a long drive, the temp goes down and regulates to 170 - 180 and slowly goes back and forth in that range. Outside air temp is 45 degrees. The temp stays there for the rest of the drive. Sometimes, the temp will regulate to 195 and stay close to this. The coolant temp gauge is accurate, wiring is good. New water pump, radiator and heater hoses, belts, no air in the system, stock coolant reserve system with a correctly operating NOS radiator cap, and I'm running a 195 degree Stant Super-Stat. I thought that with my thermostat, the coolant temp should steadily climb to 195 and stay there the entire time, regardless of the outside air temperature. Why does it initially overshoot, then vary as I drive it? Everything is very stock and in excellent shape. The engine runs fine. Any ideas? If you think I need to replace the thermostat, who makes the best one? The Stant unit that I have in now doesn't seem very good. Slant Six Dan, please weigh in. Thanks - BOB