Nissan Patrol 2001: Variable idle speed?

THE idle speed on my 2001 4.8-litre dual-fuel Nissan Patrol can be anywhere between 1100 and 1800 revs. Occasionally it will idle at the normal 750 revs and sometimes not at all. When I stop, the idle will race up to about 2000 revs, then find its way back down to 1100. The problem occurs whether on petrol or gas. I've had the car to a specialist tuner who has improved things a little, but has ended up referring me to the local Nissan dealer. Both power and economy are acceptable. Both parties I've spoken to think it's the engine management system.