I have a RAVENHEAT RST 100ET that is very intermittent. Whether on Heat or Hot water, the Fan and motor run fine but doesn't always fire. After a couple of more tries it still may or may not fire. If it doesn't the red light comes on. After resetting, again same situation. Sometimes it will work fine for a couple of days, at other times after resetting some 20 times, it will suddenly work ok. I don't hear it firing either. Relay 3 just will not operate. Very occasionally it will fire and light for a split second, then fail again. I've replaced both thermistors and I've checked the air-pressure switch by blowing into the pipe that goes into the flue and it works fine. Got a second hand tested control board this morning, but same issues persist so I assume board is fine. I can't find a circuit diagram on line that shows exactly the same board. Is there any way of checking the gas switch and the electrode cables? Any other suggestions are most welcome!