Gents, on my 2013B I experienced that stumble only on one occasion or should I say for one tank of gas. I have always run premium fuel and after reading other threads I decided to try Mid-Grade gas and the stumble appeared and stayed till I filled up again. If you haven't tried Premium fuel exclusively you might just find the solution there. I do not get any stumble warm or cold, low RPMs or high. Just Sayin.