My 99 XJ 4.0 has developed an annoying problem. It wont start unless I hold the gas down slightly. It wont remain running unless I maintain pressure on the gas pedal. At stop lights I have to hold the brake and hold slight pressure on the gas pedal to keep rpms around 1000. It used to stop when it warmed up to operating temp but now it will do it warm or cold. A friend said he thought it was a throttle position sensor. Any ideas?