If you're using an igniter that's wired to a flightbox panel that has a rheostat on the plug circuit, check the voltage it's tuned to. There should be a meter that shows what voltage is going to the plug. Since the plugs are going out in the air, this probably isn't going to show a lot of voltage if it's the cause. Most times, when it's the cause it happens on the ground. And a lot of times, it's when the plug wire is connected to the starter connectors. And that'll blow the plug RIGHT NOW.

There is also a minor possibility that the engine has somehow wound up with too much compression from the factory. If they usually have a head gasket, maybe it was left out on assembly. Some manufacturers used to fit up engines with a varying number of gaskets to get the design compression rating from parts that had maxed tolerances. You'd see one engine with one head gasket, and might see another with three.