I have an '04 F150 5.4 with 111k miles on it. I've had some health issues, so I haven't been driving it a lot lately. Anyway, it has started surging lately at slow speeds - parking lots, 25mph zones, that sort of thing. You can see the rpm vary, and feel the power change pulsing thru the gas pedal. It only happens at slow speeds - I've never had it happen over 25. Any ideas?