Any heating and cooling Guys out there? I have a question

After I cleaned the furnace it worked perferct for a day and a half.Then it started doing the same thing.I had a Heating and Cooling Guy come out twice and it still isn't working right.The Flame goes out and when the furnace does run it shuts off before reaching the temp set on my thermostat.I am going to go and see if I can find a flame sensoe today and replace the old one.