try the coolant temp sensor for the engine management it is under the throttle bodies and the throttle position sensor. mine had similar symtoms i fault code read mine several times and nothing showed up untill i pluged in the code reader from stone cold overnight and it showed up. these are the two common faults i have encounted with the 2.9EFi. temp sensors are around £15 and throttle pots around £50 if i rember correctley
hope this helps
cheers nick.