I have owned 2 boats wiht A4s. I had one with the very symptoms you discribe. It turned out to be carburator ice. I ran a foil tube from a warm place on the exhaust to the carb intake. Never had the problem again. Might not be your problem but would be worth checking out. Hang an electic blow drier in front of the intake and see if the problem goes away. Good Luck in any case.
P.S. I think the A4 is a great engine!!!