My I/O Volvo has a shift interrupter and it is connected right behind where the throttle cable connects to the carb, mine did somewhat of the same thing. It is about 1" sq box and what mine was doing as I put it in gear the box would open up and not make connection. I tie wrapped it together and it is still working fine.

i have a quadrajet carb and it was just rebuilt 2 months ago it was the first thing we did.

After the interuuprter switch---things on carb to check--even though just gone thru--crap happens---float level(should have a brass float for sure-if not --put on in) Accelerator pump( not sticking and not returning to the up position., and main rods to make sure they have not come lose.

Maybe a bad guess, but what about water intrusion into the pistons and burning them away. I know my Evinrude is an outboard, but that is what happened to mine. Same scenario, runs good on land but put in water on load and sputters out. Turns out I lost 1,3 & 6.