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Clois - if you do indeed find that your MSD box is faulty, then something on your car is killing it (them).
One thing that MSD boxes do not like is voltage spikes on the supply. They do list a large capacitor, which sits across the earth and (permanent) +ve supply lines, close as you can manage to the box itself. This smooths out any voltage spikes, and is a particularly good idea if you have a battery isolating key switch.
Might keep box #4 in working order.
HTH
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Wilf
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