1500rpm Range Rover HSE 4.6

Hi People I live in New Zealand and have just fitted another engine in my Range Rover to replace a cooked one.
Starts and idles good as but will only accelerate to about 1500 rpm then dies, backfires and will not rev up any higher.
I have searched online for solutions, changed and checked almost everything I can think of with no change.
I am leaning towards a fuel problem and have been unable to find the fuel rail pressure other than one that told me it should be around 32psi at idle which it is.
Fuel pump was replaced about 18 months ago.
Any advise or suggestions would be welcome.

Welcome on board Kiwi - you will do better posting in the RR forum: https://www.landyzone.co.uk/forum/range-rover.10/
Give 'em time to wake up and good luck. There's a host of good knowledge and experience here. By the way, at 1500/1600 rpm I'm generally cruising at between 57-60 mph on the flat but I guess you've been there.

Welcome to the forum

welcome to the forum

Thanks for the welcome guys