I don't believe you'll find the problem at the MSD box: Reason; because the entire car goes dead...(lights - horn -etc - no connection to MSD)
As I mentioned earlier the ignition switch is a good idea because it can shut down the car, But can it shut off you lights and horn? - If not - go back to the battery and grounds ideas.
When it dies again, while it's dead, gently put a jumper cable from the "-" battery terminal to a good ground and see if that circumvents the problem...if not...
I would loose the Optima Battery...(only because SO many people have had problems with them)
The MSD has an internal fuse I believe...
18 volts??? This points to a battery issue in my mind...Full charge on an Optima is 14.8 volts...
Check this out:
www.vdcelectronics.com/optima/Optima-type_NL.pdf
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