Post #4 explains it. Not exactly sure what causes it, could be a design flaw in the cap, could be inferior materials were used when manufacturing the cap, could be poor insulation of the "runners" in the cap. If you look at the cap, you'll see that the plug wires are not attached to the cap in the firing order of the engine. This was done in an effort to eliminate crossfire inside the cap, (which will cause P0300). If the insulation is inferior, crossfire will occur. If you check around the forum, (not only this one) you'll find more than just a few instances of AZ caps, and other aftermarket caps, causing problems. AZ by far is the worst offender though
Sometimes they work for a day or two, sometimes they last a few months, and sometimes they fail right out of the box.