Check the cat, it is glass inside. High RPM's would cause more backup of exhaust, and engine malfunction, from an internally broken cat. Off roading and shaking, that glass cat, could cause it to break internally. Maybe a few rocks, landed on it, while driving over them, when off roading. It may seem like a fuel problem, but actually it may be an exhaust problem after burning the fuel, caused by a internally broken cat. Especially if you have a engine check light, this is the first sign.
Shake the cat, and make sure it isn't jiggling inside.