Car fails to accelerate past 30mph under normal driving conditions. Increasing pressure on gas pedal at 30mph yields no response from car. When car hits passing gear, finally accelerates past 30mph. In addition, at interstate speeds ie. 70-75 mph car develops almost a stutter and cannot maintain speed and starts to decelerate. Both problems occur in both drive and overdrive. Has been in for repairs to 4 different repair shops, 2 of them authorized Buick dealers. They have replaced throttle position sensor, cleaned fuel injectors, new fuel pump and fuel filter, new knock sensor and blown carbon out of engine twice. Car still does not accelerate as described.

- Mt. Pleasant, TX, USA