Your plugs may be a tad cold, and your tune too rich. The engine may still have great power, but plugs foul in time if they cannot burn themselves clean.
In my picture there is the narrow black mixture ring, where the isolator meets the plug metal housing. The tune of the pictured plug is slightly lean, but that is the best tune if cooling is OK. A wider black ring is better, but it should not exceed 1/4 of the isolator length.
If in doubt, dismantle your plug to check how it looks. Plugs are dispensable, engines are not.