The pilot won't stay lit. When it calls for heat, sometimes it will runs its cycle for an hour, 2 hours, sometimes even a few days without going out. But when it does go out, it takes a lot to get it lit again. And sometimes it lights, we turn the heat on, it clicks, & then goes out again. We replaced the thermastat, the thermal couple, the filters & cleaned out all the vents. What is wrong?