The reasons for fluctuations in water temperature can be complicated and will likely require a Broomfield, CO licensed plumber.
It could be something as simple as sediment build up in the water tank. However the issue could also be corroded or clogged plumbing pipes. There is something called a thermocouple which is in the water heater and it could be damaged.
It might also be that the hot water shut off valve is defective. So you can see there are technical possibilities and so we strongly advise a licensed plumber.