I would first get a genuine, or borgwarner thermostat. Another friend of mine bought 3 new mahle units, all of them were faulty. Swapped it out with a borgwarner unit, all works as expected. I suspect the electrical heater in the mahle thermostats does not work as intendedJust boiled the old thermostat, it worked just like it's supposed to Opened just before boiling
Ac would have to be off, with the AC on, the fan should spin almost all the time(but not at 100%), especially if you drive slow or are standing still.This made me wonder. Have you ruled out the fan being on (or on full speed) all the time? On the manual cars it should only come on when needed, commanded by the DME. If you have INPA there's a test you can do where you tell it to spin at 0%, 25%, 50%, 75% or 100%. If you step through those with the engine off you'll be able to hear when it's on (or you can get out and check) and the pitch will increase with speed. Or you could just start the car with the engine cold* and make sure it isn't spinning.
*question to fellow fanatics: would the AC need to be off too?