nissan almera 1800 coil pack failures?

Hi, I have problems with the coil packs breaking down when starting from cold, it never happens when the engine is hot and no symptoms experienced in running mode. I have now replaced two coils in a one month period, the last one lasted only a week before it broke down again, the supplier said it could be faulty and replaced it, but after refitting it the engine now refuses to start, firing on two or three cylinders only.
I have checked the timing chain for stretch and this appears ok, the depth of the plunger on the tensioner is 7mm at most. A hand held computer analyser shows a fault on the crank case sensor, which I intend to replace, but I do not understand why this should cause the coils to break down. Has anyone had the same experience, can you help me with this problem?