Having the same problem with keyboard. I'm a complete noob using X-plane 9. Just now using it again after 3 year hiatus. F2 starts throttle up which continues until full throttle, F1 starts reduce throttle until all the way, uhhh, un-throttled(?). I can find no combination of tapping, holding, or other buttons that will stop the continuous change in throttle. It seems to also be a very rare problem.