Had the same problems.
Checked thermostat, sensor, flushed the coolant, etc.. no change.
Started to worry about blown gasket.
I finally got the radiator to a specialist who cut it open, cleaned the silica deposits inside with a metal blade (had already done a chemical cleaning but no luck) and soldered it.
Since then my temperatures are back to normal!!!!!
Seems like the 636 radiator has too thin ports that clog easily.
Have your radiator checked by a specialized workshop!!!!
And then Little red riding hood bought a Kawasaki with grand-mom's insurance policy and lived happily ever after!!!!