I had an '89 5.0L mercruiser that would cut off on me at various times. Somedays no problem and other days would just quit. I narrowed it down to the ignition coil, however, during the testing of, it passed.
Didn't matter if coil was hot or not....it would just quit at random moments. Never understood why, but just ended up replacing the coil, and all was well! Never had another problem!