I have a 2005 3.8 liter impala with 250K miles. If I keep the car under 3 grand it seems to run fine. It gets 27.8 mpg driving to and from work. Although the hire the RPM's it acts like a clogged fuel filter. And at full throttle it will not accelerate it bucks and chugs and just won't go. Just like it has a clogged fuel filter. Things I have done to try and fix the problem. First a new fuel filter. No luck. 2nd a new converter. Still no luck. Fuel pump key on engine off over forty pounds of pressure. Same when it is running. If you rev it up the fuel pressure jumps above fifty psi. When you shut it off pressure holds so i am assuming the Fuel pressure regulator is good. This meets the specs for the pump. I have inspected the MAF sensor. No damage seen and the scan tool data looks good and smooth. When I put the pedal to the floor the scan tool goes nuts. It goes from closed loop to open loop, best i can tell all sensors start jumping around with their readings even the fuel injectors pulse width gets erratic. But if you let up to say 98 percent throttle all of the readings calm down and the car starts to accelerate all though it is slow to reach speed the more you let out of it the faster it picks up speed. I hate guessing as it seems to cost a hundred buck every time i do. So if you have any ideas please let me hear them. Thanks in advance.