Loose connections or clogs in the lines are usually the cause of intermittent operation. Faulty control switches will produce the same symptoms. If u have a pilotless model, it uses a spark igniter to ignite the gas, which works in a combination with a spark switch, it might need a replacement once it burns out.  There are also possible causes such as defective thermostat, defective selector switch, and defective gas safety valves which needs professional care from our technicians.