Welcome to the Ford Coil club. If you've only replaced 2 bad coils, you only qualify as a Junior member. Bad coils are a VERY common problem, especially if they've gotten wet. Also check the heater hose to make sure it's not dripping on the coils (also common). I've seen several of these that had to replace all 8 coils. I told a friend that and he refused to believe me and replace them all at once. So he ended up replacing them all--but one at a time as each one went out. Cost him a fortune at the shop since they charged him minimum labor each time. He's a slow learner.
BTW, most coils fail when they're hot and under load--exactly what you're describing here. | 
