water heater temp. changes

Recently my gas fired 40 gal.water heater has had water temperature problems. In order to get water temperatures Ive been getting for aprox. two years with my existing water heater I have to turn my temperature control up quite a bit to hot. This problem has happened with previous water heaters when they are about two years old and I think Its because of mineral deposits collecting on the temperature probe in the tank keeping it from detecting the correct water temperature??? How can I correct this problem and keep It from happening in the future.