I have a mystery on my hands, hopefully someone can give me an answer.

My '99 S80 T-6 started idling and running real rough. I got an associated emiision control msg. and a flashing check engine light. I pulled a coil off each spark plugs one at a time and found one not producing a spark. I went to Volvo parts and bought a new one($60.00) , installed it and the car rode great for ten minutes. Then the same problem occurred with the rough idling etc.. I removed all the turbo tubing again an found that another coil went bad. Back to Volvo to get number two coil. I installed that one but this time I still had the rough engine. Day number two, I am refreshed and thinking clearly. I find that two other coils are bad. My question is why did all the coils work properly after I installed the first bad one for about ten minutes. I already moved the last two bad coils around to see if it was not the wiring. If I buy two more coils, will they keep on shorting out for some unknown reason. I feel I am so close, I would hate to bring it to the dealer, after all the work I have done. Why all of a sudden are these coils going bad? The car has 115K on it.

Thanks and the pressure is on since my wife needs the car!