What makes my thermostat kick on and the Flames will start heating the house but then shut right back off and the fan stays blowing set the thermostat at 75 and it'll only go to about 60

Hi Adam, more than likely one of the build in safeties are shut off the flame. If you post more information, make and model, you should get a better answer. Good luck

One of the heater safeties tripping sounds likely. There are a number of things that can cause it to happen. Restricted air flow, collapsed ducting, dirty filter, flame sensor, roll out switch, high temp cut off. Unless you are experienced with HVAC, aside from filter changes, it is best left to pros.