Well at that kind of mileage all boots should be replaced and make sure dielectric grease is used on new ones.If you look at one of the coils that you replaced what was the ohms measurement of it.I bet you would see a open or a very low resistance.
Inside the coil is a capacitor that goes bad.(high heat)this cause the misfire.
You should be thankful you haven't had a plug blow out.
Because when they blow out the wreck the threads in the head plus destroy the cop.
Chk all your boots
Rich