I have had several ignition unit failures and all were intermittent. One time the plug boots got soft when they got hot. this caused mis-firing, especially late in the flight after things heated up. A similar failure occured when the module itself got hot. One cylinder would drop out. after a few minutes of cooling, it would work again. Another unit had intermittent failures where the engine would just quit in the air. I fought this one for a few months before swapping out the module and it has not quit in about 30 flights since then.