i bought a boat last spring and it had a 60 merc on it that had been worked on and it did the same thing and i found that the fuel line was being pinched shut by the hood sometimes not all the time but when i would get it home and check everything some times i guess it would get pinched took me awhile to find it but i shortened the hose and works fine now . might even want to check your fuel filter if you haven't already .

scott