I have a single loop boiler. I think its about 40 years old and it won't stay lit. I replaced the thermocoupler many times. The new thermocoupler would last from days to weeks before the boiler would go out again. This happens in any weather conditions, Cold, not so cold, windy, calm. The flame is covering the themocoupler pretty well, but I can't turn up the pilot any more. Any Ideas?