I have a 2000 grand prix gt 3.8L (not supercharged). While accelerating the motor stumbles around 2000 rpm and while holding the gas steady it fluctuates from about 1800rpm to 2300rpm. I do not have any check engine lights and when I let off or accelerate it will stop. I have unplugged the EGR valve as suggested by the Haynes manual but the problem continues.... any ideas? Thank-You!