98 JEEP GRAND CHEROOKE NOT GOING OVER 40???

Hello! I own a 98 Jeep Grand Cherooke 5.2 LSE. The jeep has
120k miles. My problem is that it will not go over 40 MPH. You hit
the gas and it just reves and sputters and will only really shift,
only into the smaller gears if i drive slowly and let off the gas to
shift. I took it to the transmission shop and they said the tranny
was fine, they got error codes off the computer for miss fires from
certain cylinders and a bad gas cap, as well as a faulty front o2
sensor. I repalced the gas cap, put in new plugs and plug wires along
with a cap and rotor and then replced the front o2 sensor before the
catylytic converter. The problem is still there? Any help would be
really appreciated, i cant even go on the freeway and am aftraid it
will go out on me, Thanks STEVE!!!!