Dave,

Does it crank strongly? The only things I can think of are that you are connected to shore power and keep the battery charged but have an old battery that will not hold a charge and/or a bad alternator. I found out about a bad alternator that way. The previous owner kept it charged up with shore power, but I sail off a mooring and depended on the alternator. I had no idea that the alternator wasn't working until I took a long sail and found that the battey was barely able to start the engine. Running the engine longer only made it worse. Depending on the battery condition, one start could be enough to drop the voltage to a problem level.

If the alternator and battery check out, I can only imagine that the old engine is just offended by all that sailing.