Furnace intermittently turns off. Lennox G23Q23-75-2
 

My Furnace will eather not turn on or turn off before the set point is reached.
When heat is commanded the small fan turns on then about 15-20 sec later the igniter will start to fire then a short time later pilot light comes on but the main burner does not.
I noticed that when it does all work OK, I hear a click a short time before the main burners turn on.
I also noticed that the time that the igniter is on before the pilot turns on vary alot. I think it is much shorter igniter time on time when the main burner fails to turn on.