Our yacht club rescue boat has a 50hp merc, which I am convinced is just a yammie motor on a merc leg. It had the exact same problem as you describe. Anyway, there is a low pressure fuel pump on the back of the motor, exactly the same place and type as my 50hp yammie. One of the guys at the club pulled it off, tested and seemed ok, put it back on and now the problem is gone. Not sure why, maybe it was sticky or something stuck in there.