I've got a 97 Ck2500 4wd 5.7 Vortec w/64000 miles. When I get out on the highway it bucks and hesitates and it's very annoying. Usually anytime over 65 MPH. I've had a complete tuneup wires included and have had it back to my mechanic 2 times without any results. He says fuel pressure is fine and will likely look at the Catalytic converter next. Hooting around town there's no problem and it cooks, it's just the highway driving... Any ideas ?