This is usually caused by a bad Accelerator Pedal Position Sensor (APP Sensor). It is located on a bracket above the accelerator (gas) pedal. If you need a replacement sensor the GM part # XXXXX ($120.05 list)
ACCELERATOR PEDAL POSITION SENSOR VOLTAGE (4.2L)
© 2008 Mitchell Repair Information Co., LLC.
Throttle Position (1) Volts
APP Sensor No. 1 Less Than 1.1
APP Sensor No. 2 Greater Than 3.9
Wide Open Throttle
APP Sensor No. 1 Greater Than 2.0
APP Sensor No. 2 Less Than 3.0
(1) Measured at sensor or as seen on scan tool.