Bradford white question

I have a 40 gal nat gas BW heater that is appx 2 yrs old. Recently it seems like the thermostat on the unit only works appx 50 % of the time. It is set on the median suggested mark and once or twice a week it will run out of hot water and the unit will not call for heat to keep it warm. If i turn it up towards really hot it will fire and heat the tank and shut off like normal and then maybe work a day or so normally.
It there something that needs to be cleaned / adjusted or replaced that makes sense? Some kind of sensor?
It is in a relatively clean environment in a basement,(Some minor pet hair etc) but nothing crazy. I would appreciate any help/ suggestions.

Thanks
Stbentoak

As long as this is not a power vent heater it is likely that the thermostat is going bad. You will probably have to replace the thermostat/gas valve. That should take care of the problem.