It's a hard one, because it could be caused by the engine idle speed blipping higher or the transmission doing something odd. There could be a lot of possible causes for either, so it's really going to be hard to give you much specific guidance. The fact your tranny fluid looks good (and is changed regularly) is a positive though. It could be you had a bad tank of fuel that caused an odd idle... Basically, if it's not happening now, just do your best to forget it ever happened. ;-)