Hi,
the symptoms sound a bit like a head gasket problem, but I wonder that you've noticed nothing strange on the comression test. Also, the head gaskets on the PT are usually very reliable.
Regarding the sensors. One is located in front of left head, directly where the cooling fluid hose is connected to the head. This is the "primary" sensor. Only if this sensor fails (shortened or open), the sensor at the radiator (bottom) is considered. This are at least the observations I've made, on both the pre- and post-face-lift instrument cluster. Both sensors are subject to malfunctions over the years, an exchange would make sense (shouldn't be too expensive).
The electric fan is running whenever the A/C is enabled, otherwise only if the temperature exceeds specific limits (I have 98, 103, 108 degrees centigrade in my mind for the respective fan speeds). You can test if the fan speeds work properly (requires an intact fuse and intact relais) by plugging out both coolant sensors. The system will then assume a temperatur of, I think, 110°C and goes to maximum fan speed. The engine must be running for that test. You'll be surprised how loud this can be!
Important about the viscois fan is that it becomes "strong" when the engine warms up. You should not be able to stop it literally by hand (I don't have to tell you that you shouldn't really use your hand to try that when the engine is running...). If the coupling appears to not work properly, it may still be the radiator who remains too cold. Did you try if the radiator becomes hot on top and at the bottom when the thermostat isn't built in? If the radiator is clogged, heat distribution is usually unequal.
Regards
Bastian
