Range Rover 2001 2.5 DHSE Fluctuating Revs in D, N and P

1st time Poster,
I am trying to help my father-in-law resolve a problem while not being ripped off!!
His pride and joy, has an issue with the revs fluctuating to 1300 rpm while in D, N or P but not all the time.
There are no fault codes on the dashboard.
The mechanic has indicated the gearbox and so we have gone there but he cant log onto the VIN using 3 different diagnostic tools or even the engine!! and suggests its an electrical issue or a mechanical one.
Anyone ha these symptoms before or can point me in the right direction. I just don't want to burn money in different garages with no solution!
Grateful for any experienced suggestions.

Welcome to landyzone
if you post your problem in the range rover section help will come, you will not get better help for a p38 diesel anywhere else
https://www.landyzone.co.uk/forum/range-rover.10/

Welcome to the forum

Welcome to LandyZone

Make a ‘thread’ in the above-mentioned section, they will know best course of action. Not many diagnostics work with P38’s I’m afraid but there are a few, and even a section on this site where people that have diagnostics near you may be able to help

Good luck