Look for the simple things like: Is your throttle/brake linkage binding preventing your servo returning all the way? Whats the condition of your throttle /brake servo? Is it old an worn, is it working properly? Did you by accident (or not) possibley hit your throttle / break trim on your transmitter? EPA's set correctly? If it was once running smoothly and now just "started" doing this without you adjusting the needles those are some examples of where I'd look right off, unless you have had a big weather change where you have to adjust them for proper tune.