Could you remember on that time you got no fire, if you smelled gas out the exhaust just for a second? You gotta smell gas! Just hearing the valve click isn't enough, as you may have a gas delivery problem, if you don't smell it. IF YOU DID, that tells you that the gas valve opened, but there was nothing to ignite it! The times that it did light, the HSI was able to make enough connection to glow, then once it heated up a couple of times, the crack opened up and no longer was shorting (glowing). It is probably the HSI. Take it out, and if it has a white water-stained looking mark, theres your crack (you cant always see the crack). you can double check by turning off main gas to heater and try to fire with HSI out (careful in case it begins glow). If it does glow, shut down the heater and repeat two, or three, or how ever many times it took last time before it wouldn't fire.
The flame rectifier is part of the HSI so to replace one, you're replacing both.