The OEM coil packs are a common problem on these cars. Don't assume the DTC codes (or lack thereof) will be able to identify the problem with 100% accuracy.
The funny thing is, the Ford replacements appear to be highly reliable and are a different part number so they seem to have corrected the issue. Definitely replace them all, attempting to just change the "bad" ones is a waste of effort. They are all the same flawed design. They seem to burn through the plastic case and arc at the output.
Lifespeed
2004 Silver Birch Marauder 130K miles
Wilwood Aero6 F, NMDP R brakes, Penske 7500DA shocks, Hypercoil 600lbs F and Grand Marquis R air springs, Addco tubular sway bars, Metco control & Watts, 31 sp axles, Stainless Works cat-back exhaust, Lidio tune, American Racing AR883 9.3" 50mm F 10" 59mm R 20" wheels, Pirelli P-Zero 265/35-20, 275/40-20 tires
Coming soon: Livernois 5.0L stroker, ADTR/Vortech V2 intercooled SC, Silverfox trans and Circle D converter