I'm thinking the problem is fuel-related. If this is the case, I think there's a reasonable chance that it has to do with either the fuel "stratifying" or that some crude is coming loose and mixing with your fuel after a long sail or motor sail. With the boat heeling, or just boucing through wave action, the stirred-up fuel could cause it to burn differently in the carb. The ethanol in the fuel could also be a contributing factor. I've had luck adding regular fuel stablizer. It takes a while for it to really improve the performance, as I've had to burn through all the old fuel first.