rough 4th gear and shut down (97 4.0 5spd 2 dr)

any one know why my XJ has one heck of a rough shut down? it doesn't seem to have enough "UMPH" to it... I just changed the plugs, cap, wires, rotor, oil & filter and it's still sluggish. It's 100% stock with no intake/exhaust and it has a newer clutch. the XJ its self has 140k.
In 4th gear it seams to sound rough while accelerating into 5th.. a low pitch roar. I just ran sea foam in it today too... but the problems have been occurring since i got it last week...

any one?