Champion Generator 4000/3500 trouble

Bought A New Champion Generator used It From Sep Though Feb Then Again Last Week. I've Not Had Any Issues With The Gen Until The Last Few Times I've Used It. The Problem Is The Gen Surges And Gallops Bad And Then Will Stall. I Check Fuel Bowl Lines, Air To Tank Line. No Trash Or Debris. Always Use Fresh Gas And Start The Gen Every Two Weeks. It Has 130 Hrs Of Moderate to Medium Load Hrs. Use it For My TT. Now, This May Sound Strange, But When The Gen Is Off And I Open Gas Cap For A Minute, The Put Cap Back On, it Runs Great for a Couple of Minutes, Then Goes Back To Galloping. Is it Possible The Gas Cap is not venting right? Model Number Is Champion 46598. Thanks.