I have an interesting problem that is proving difficult to diagnose. Carrier 8000 Weathermaker 58WAV075, problems with intermittent igniting. Every so often it fails to ignite. When it does fail, it goes thru usual process, ignitor glows but valve does not open. No flame, then code 34 "ignition proving" error. Eventual code 14 lockout. If wait, turn off main power and retry it will often work again for some time a week or more. I've done the following with no success - cleaned flame sensor, cleaned draft safeguard switch, cleaned pressure switch, pressed reset switches on sensor that have them, and tested for 24v where it is supposed to be present. Then, since I have an identical twin furnace, one by one I switched out the controller and all sensors. Confirming each time that it does not fix the problem, and does not create the problem in the twin furnace. All I have left to test is the gas valve. I'm beginning to suspect this is the problem. Is there something in the valve that can stick or cause an intermittent problem such as this?