I had a 75hp mariner same age and is the same as the merc. I had the same proplems, i managed to trace down to two things either the triger switch or the switch box. The two lower plugs on my engine had no spark changed triger switch but still the same. I then changed the feed wires from switch box to top two coils too the bottom two coils.The bottom two then sparked and two top didnt so the switch box wasnt powering up for two of the coils. I fitted a second hand switch box and it solved the problem.
Having same problem I am, sounds like. I had my '84 75HP Merc rebuilt last year. Ran it first time last weekend. Ran solid and strong almost 2 hours, but then as we approached the ramps to unload, it start choking down like running out of gas. I still had 1/3 tank remaining! I can recrank and it will run 2-3 secs, then power down. Over and over. I'm looking online for someone that has experienced same thing so I can fix and get out on lake couple more times. Tony
