Car hesistates in 1st and 2nd gear. Have replaced the following: O2 sensor, spark plugs and spark plug boots, ignition coil, throttle body, shorted out fuel injector and fuel pump. Car still hesitates in 1st and 2nd gear. Has been doing this since this day 1and car now has 29,000 miles. Gas mileage far less than rating. Chevy cannot get a code on the computer so they dont know what is wrong. Can you please help?