15 years ago I had a 55 hp evinrude that ran very rough . Repair shop
did a tune up but could not find anything amiss. Next time out same
thing happened. Once up on plane , hitting wave tops , engine began
missing and stumbling. By accident I jiggled ignition key , found it
was loose. Turned out ignition switch was making/breaking in rough
water.
First thing I check now on every trip out.

Keith in BC