I own a 1997 Jeep grand Cherokee with a 4.0 L and at random times it will go down to about 250 rpms, just enough to still have power steering and then it will come back to normal rpms and it will back fire and blow of the muffler completely. I have replaced the pickup and the crankshaft positioner. I've checked the fuel pressure at the fuel rail and it is 46 psi. Anyone have any idea what might be wrong?