yamana 01 fourstroke 50 idles rough,then smooth

[nashfireman]

01 fourstroke 50. when in foward gear at idle this thing will run smooth as silk for a while. Then all of a sudden it will run rough for 30 seconds or so. Then smooth again. It will do this for back and fourth for hours while trolling. But, its always smooth when not in gear. Iv changed the plugs and saw no difference. Ran seafoam carb cleaner in the fuel and that didnt help either. Where should I look next???