This could be either spark or ignition related….Does this happen more on warm, humid days? Could be the infamous vapor lock…I might give a try running it with the cowls off and see if the situation gets any better, just as a test. Later Mercs similar to yours, have water-cooled fuel pumps. You’re not using winter blend gas, are you? Do you try pumping the primer bulb in the fuel line, when this happens? Vent on the fuel tank open? When it’s in fail-to-start mode, if you have a nice, quality, pump oil can full of pre-mix gas, you can squirt it directly into the carb opening and try to start right away. If it fires off briefly, it’s fuel related. I do this all the time without incident; but be sensible and have a fire extinguisher handy, JUST in case.
It might also be an ignition part, such as the coil in the distributor, failing when it heats up. If you have a spare distributor to try, I’d try that, too……
EDIT!!! Don’t drive with your wrap around cowl off, and then come off a plane and fill the lower cowl with water! You boat is a style that will probably do this!
Long live American manufacturing!
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