Sounds like your PPS (pedal position sensor) is starting to fail! About $90.00 for the part and you can replace it yourself, very easy! http://www.autozone.com/autozone/parts/Dorman-Accelerator-Pedal-Sensor/2003-GMC-Envoy-4WD/_/N-jk7bbZ9guhk?counter=0&filterByKeyWord=pedal&fromString=search&itemIdentifier=940660_198232_0_