Could be the sensor is bad or it is running hot . Is the factory coolant ? I would change it out if you have not already . I use Prestone premixed that says safe for aluminum . The motorcycle stuff like the pink or blue ( forgot brands ) is a fortune . Check the coolant level in the radiator and reservoir when cool . Some can get air in the hoses and need to be burped . Remove the radiator cap , start it up and let it idle until it gets good and warm . Squeeze the hoses and see if any coolant comes out of the radiator . Make take a few squeezes to get the air out of it if it has any .
How is the oil level and color ? If low or blackened change it out and see if it runs hot .
Any mods like airfilter or exhaust ? One that runs lean will run hot . Do a plug chop and compare with this chart
http://www.4secondsflat.com/plug_chart.html
On my Echarm the cooling fan is powered by the stator instead of battery so if I shut off the engine while the fan is on it dies . I have done that and went to restart it and the fan had a draw that made me have to attempt to start 3 or 4 times instead of normal instant start . From then on if the fan is running I let it idle until it shuts down unless I will be parked long enough for it to cool down on its own .
Does the temp gauge have numbers and does the fan come on around the same place on the gauge ? Make sure the radiaor vents are free of and debri and that they have not been bent or mashed .
A hydrometer is a squeeze bulb tube with balls in it and you count the number that float when you use it to suck up a sample of your coolant in the radiator as it is impossible to eye and guess if its 50/50