I have an 01 F250 PSD w 6 spd manual. Just today while towing my dump trailer, I noticed something which makes me wonder if I'm gonna need a new clutch.. Especially in 6th, while towing, when I accelerate hard, the rpms will (sometimes, not consistently) rev higher, then lower on its own, almost like its shifting into a higher gear on its own. Is this a clutch issue or something else? If so, ball park on what a new clutch will cost me?