I'd go for the MAF sensor first for these symptoms, i've seen cases when generic OBDII readers gave erratic readings on D2s, even though the petrol engine should be OBDII compliant the diagnostic protocol is a bit LR specific

Originally Posted by
ENGINE MANAGEMENT SYSTEM - V8; 18-2-22 DESCRIPTION AND OPERATION
In the event of a MAF sensor signal failure any of the following symptoms may be observed:
l During driving engine rev/min may dip, before recovering.
l Difficult starting.
l Engine stalls after starting.
l Delayed throttle response.
l Emissions control inoperative.
l Idle speed control inoperative.
l Reduced engine performance.
l MAF sensor signal offset.
Discovery Td5 (2000), manual, tuned
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