Howdy. New From Texas

[txallen6607]

I have a 2003 Chevy Silverado 123600 miles. Runs like a top.....until a few weeks ago. It has a 4.8 L engine. When i accelerate (GET ON IT) It starts off slow and then builds up and then begans to go faster. Before, i could floor it and it would take off very well, sometimes spin the tires. Theres no hesitation, just feels as if its "bogging" down a bit. Today, i got it back from the mechanic and it id still doing the same thing. I chose to take it out because they had it a week and still didnt fix it. What they did to it is : clean the injectors, new spark plugs, check cadalitic converters (still good), change air filter, clean throttle body intake, replace fuel filter. They said that the only codes that were showing up on the scanner were o2 sensor and knock sensor, but they explained that those 2 things would not cause the problem i am having. I am at a loss as what to do. Sorry this was so long, but it was kinda hard to explain. Any help would be great.questions will be answered as best i can. Thanks in advance.

ps, when i try to pass someone that is going 50, it takes forever to get up tp speed to pass, IF i ever do get to pass.