Started stalling out and became extremely hard to start?

1991 Toyota Camry, mileage: 139,000. My car started stalling out and became extremely hard to start. I took it to our excellent service shop (I trust them very much) and they replaced the ignition coil and igniter at a cost of about $565.00. About three weeks later it stalled and needed towing and it was the coil that had gone out. About two weeks ago, the same thing happened except they determined that the igniter is taking the coil out, so they replaced that again. All of this is under warranty so I don't feel too bad, but now it stalled out again, in the same manner. Are they just getting bad parts or is there something that could continue to take out the coil and the igniter on a regular basis. This will be the forth coil and third igniter. Thanks much for any clues.