Honda ex350 generator rev changes

I inherited a honda ex350 generator. It is one of the ones that have an alternator and a built in invertor. It serves the purpose I have for it whichis to charge some batteries and run two fluro lights on occasion. But the engine keeps self adjusting its revs, it will drop to very low revs then sound like it is surging up to high reves, it does this continually and although it is quiet the continual change in noise is annoying. Does anyone have one? and does anyone know if this is what it is supposed to do??