My 97 f150 4.6L V8 has repeated quad ignition coil pack breakdowns. They are lasting an average of only 4-6 months. I have replaced these several times. Failures have appeared on both driver and passenger sides and the average lifetime is decreasing. I strongly suspect that another igntion system problem is causing the coil paks to break down. I have replaced spark plugs and wires. How likely is it that the voltage regultor or alternator is defective and overvoltage stress is ruining the coils?

 

Do the coil packs have the little condenser like attachment onthem? If it does, I would look closely at them.

 

I have only seen this type of problem resulting from bad plug wires. Are you sure the replacement wires are good? Ever buy a brand new defective part? Check the resistance of each wire with an ohmmeter. Any measurement greater than 6,000 ohms is definatly bad.
Also, the obvious. Be sure the wires are completely seated in both the coilpack and the spark plugs