My '89 420 SEL had the same problem and it was the coil. Drive for a while and it would quit and not restart until it cooled. It took two tries to get the correct coil, too, as the parts books seem to be confused.

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Gregga
'89 420 SEL "Moby Dick"
'30 Ford Coupe
'31 Ford Slant Windshield Town Sedan
'33 Ford Pickup
'34 Ford Pickup
'01 Honda Passport