i have a 2007 hd sportster and when i ride for a while my engine will stay reved up. sometimes it not as high and it goes back down but sometimes it reves pretty high. i had the cables lubed but it still does it. not as often but still does it

 

I'd suspect that one or both of the cables is unraveling inside the housing, or maybe the grip needs lubrication...you might also want to post this in the Sportster section...