Coils ?

I have a 98 expo with 95,000 miles and about every three months a coil goes out. Its always a differant one. I think I replaced 6 in the last year and a half. Do these things just start going bad at 95,000 or should I look for a nother problem causing it.

I have a 98 and from what i've been told they start going bad around 100,000. Which if the new ones last that long it won't be to bad of a deal. Hope this helps, Mike