I have a 2001 Defender with same issue. In my case, I did 4 things alltogether and it solve, so I don't know what was the error...
High / Low switch in transfer was put in High all the time
IAP cleaned (was as bad as yours)
Traced a short in the wiring loom that goes to the IAP at manifold, fixed with insulation tape
Disconnect the MAF
Never have had another bad response issue, sometimes it gets kind of slow but with only cleaning the IAP it goes perfect again.

