need more info. what type of vehicle, what type of engine, its configuration in the engine compartment, ect, ect....
you are absolutely sure that the only vehicles that exibit the voltage drop are the ones that are blowing coils?
first thought was, they all get a voltage drop, most sooner than later. connections get bad/oily, things corrode, fasteners loosen. it happens.
but if this is only on the problem engines, it may be that there is something in the coil packs shorting out the +v lead to the block somewhere, rasiing the potential of it, making it look like a drop if measured while running (therefore power ti the coils)