2001 Jaguar XJ8 • 102,000 miles

Since February this year, 'Restricted Performance' Indicator and Engine Management light issues. Have replaced the Air Intake Box, Air Filter. Have had the MAF Sensor cleaned. Fault codes always the same: P0171, P0174 (Lean on both banks). Now getting fed up! RP Light comes on when engine is at normal running temperature and under load, when load reduces light goes off and stays off until the engine is stopped and started again when the same happens on the next trip. If revs increase with light on, a lot of ' shuddering' as the engine misfires regularly. What do you suggest next steps? I love this car but my patience is wearing thin right now after several trips to my (very good / reliable) mechanic and a more expensive one to my local Jaguar dealer. Not sure if relevant but I use Cruise Control a lot as I do a lot of motorway miles?
Ta, Russ.
October 22, 2012.

P0171/P0174 -Could be a vacuum leak, If it has an EGR have that checked too also check fuel pressure could be low

Oct 22, 2012.