Coils normally just snap "burn out". Both running rich and lean can cause this.
My pinch is about 5 seconds, some people say 2 seconds and less than that and your too lean. Regarding the LSN Remember make a small change, drive it (where possible) for a min, stop for 20 seconds then boot it and see what happens? also don't forget the HSN its more likely that the car is running lean at top end to blow plugs.
How does the car perform normally when its running?
Out of interest, how do you stop the engine?
.