My (old) 36" viking with a GXV340 Honda engine has started to do a strange thing lately. It cuts great, runs great while cutting, starts fine and all that. Except. When the engine is running with no load, like when I shut off the blades to empty the catcher, it acts like it is going to die, it cuts out and stalls. It never actually stops completely, but it acts like the spark is being shut off randomly. Only when running without a load. Has new fuel filter and spark plug. Any Ideas?