Many of the things have a fairly decent troubleshooting section in their manual. Have you been down there when this happens? It is often easier to evaluate if you can see it happen. Once it does happen, do you have to reset something to make it work, or does it self-recover eventually?
Modern systems often have many levels of safety interlocks. Reading and understanding the interlock and start/stop routine of the system can go a long ways towards isolating the problem. A closed combustion system is more complicated than one with an atmospheric flue, for example. There may be flame decectors, pressure sensors, overheat sensors, etc. in the circuit, and any one of those could trigger the system something's not right and shut things down. If there's a circuit board, often there are a series of lights that show you what's going on. Reviewing those and their sequence is critical to discovering what's going on. An intermittent problem is the hardest to diagnose.
A faulty igniter is only one possibilty. Just in that one component, it could be the controller not sending the signal to ignite, the igniter could be dirty and not spark (or glow if it is that type), the gas valve could be dirty and there isn't enough gas, the flame sensor could be bad and even if it did light, it couldn't sense it.
You need to read the theory of operation and then diagnose. Not all techs are capable of this. If you can't do it, it can get expensive finding someone who can. Trial and error in replacing potentially expensive parts is not the best way to fix it, either - you've got to throw some understanding in along with it. Just like yesterday's car mechanic, unless he is smart and willing to learn, today's electronic devices can be hard to understand and old school can't cut it on these high-tech devices all the time.