OK, so you have a 1967 3hp Evinrude, correct?
Well, your engine has two coils under the flywheel, one for each cylinder. It is possible that one or both coils has failed/cracked. The best bet is to check spark immediately after the engine stalls to see if you are losing spark to one or both of the cylinders.
In order to get at the coils, the flywheel must be removed, which requires a special puller. Older coils have a tendency to crack, creating a short to ground on the secondary circuit. Your flywheel may have an inspection cover that would allow you to sneak a peek at the coils without removing the flywheel. The coils are NG if the cases are cracked.