I have a 96 cherokee sport 4.0 with the same problem. i am also a auto tech for 15 years and this problem has whipped my butt and my friends and co workers as well. I have changed or tested every ignition part on the vehicle and still the skip. the only code i get is a tps wich has been changed. Sounds like you have done the same. the only thing left is the carbon on the valve thing wich actualy makes some sense so thanks for posting that info!! Or a bad ground some where. Jeeps have a ground strap from the fire wall to the head or ive seen some to the intake ck that as well seen many problems from theese straps.