Bob 04's suggestion is spot on. Before digging into arcane stuff, check the exhaust elbow. Yanmars are famous for clogging elbows, and they are "routine" maintenance items. 3-5 years is about all you get in use. The symptoms are just what you say. Hang around Yanmar forums and you start believing that Yanmar owners might have a spare fanbelt, but they always have a spare elbow.