Could also be like mine. I have a gp460 that I rode out yesterday for a good 2 hours, barely made it home.
Occasionally the throttle on the carb (HDA223) would stick and revs were around 8k from 2k idle. I could tap the throttle a bit and it would bring back down but I was having surging issue. Then it would just go away and come back. Problem for me is I need a rebuild. Yours is probably the Idle screw or Low carb needle adjust (easy to do).
Pictures tend to tell a better story.