As mentioned above your vent line may be clogged from anything from spiders to mud from a insect trying to find a nest . Generally it is a small chrome or plastic cap like looking piece on the side of the boat or stern. Blow into it to check if it clear . Then if that checks out I would pull the gas line by the carb and check the fuel flow /pressure with a gauge of some sort. Gas caps on boats are not vented , because of water intrusion . There is no fuel pump in the tank on most boats so I don't think the gas tank is your issue ,unless it's leaking and you would smell or see it in the bilge if that was the case or your gas gauge wasn't working correctly. You should be able to follow the fill hose to the tank and if it goes under the deck and you can't follow it anymore that's probably where it is located. Now your talking about cutting it out before you verified if it's a fuel deliver problem caused by that , a clogged fuel filter , remote or in the carb ,the carb itself , or a bad fuel pump. Have you run it when this problem occurs and then loosen or remove the gas cap to see if it then runs like it is supposed to ? By doing that you eliminate or confirm the vent line it self is the culprit.