Water Heater Temperature Question

We have a 75 gallon water heater (10 years old) and some mornings the hot water is 120 F and some mornings its 102F.... Is there a way to decrease the range of temperature swings? We would like it always to be about 120F in the morning-
Thanks for the help-

You may have a significant buildup of mineral deposits in the tank. This can act like an insulator, preventing the thermostat from getting an accurate reading of the water. At 10-years, the thermostat might be getting cranky, too, and replacment of it might help. But, at 10-years, you could have another 10-years out of it, or it could fail tomorrow. Is this gas, electric, or other?

Thanks for the reply. It's an Apollo water heater; 11.5 years old. Any idea is changing the thermostat is a DIY project? I'm pretty handy....
Thanks.