Idle Falcon V8 ute

My 2004 Series 1 BA Falcon V8 ute has always had an erratic idle problem that the dealer has not been able to diagnose or fix. Fitting a Series II throttle sensor made no difference. When sitting at traffic lights with one foot on the clutch and the other on the brake the idle will fluctuate wildly. As an experiment I have let the car roll forward when this occurs and while the car is rolling and out of gear the idle will remain steady, but as soon as it stops rolling the idle fluctuates again. It's enough to make a loyal Ford owner see red.