Carbon and or spark plugs getting too hot.
Standard NGK plug for a cross-flow is a BP6ES, put a set of BP7ES plugs in, ie one grade colder running. I had the same problem years ago with two different cross-flows and colder plugs fixed it.
There is a downside in that the plug is slightly too cold for a standard engine, but it's only noticeable if you spend a long time in slow moving traffic.
