There is nothing on a P38 that can't be mended in one way or another but you do need some instruction and there's a lot of it about. You may have heard of RAVE which was the Land Rover workshop manual but it hasn't been updated since 2005. But, as the P38 ceased production in 2002, who cares.
RAVE can be downloaded from http://rangerovers.pub/static/rave.zip but it might take a while as it's a pretty big file. To run it, simply double click the rave-lr.pdf file and you're in.
There's also an online version that can be found at http://workshop-manuals.com/landrover/p38/range_rover_workshop_manual_volume_1/
The full parts manual can be found at http://new.lrcat.com/
and if you can't figure out exactly what the problem is, there's quite a few decent guides to the common faults that can be found at http://www.rangerovers.net/newrremedies.html although it is a little out of date now with some of the advice as it hasn't been updated since the original site was taken over by a Canadian company who allow it to be run by the main reason this forum has been created.
There's also a few archived articles of use that SpiggyTopes (Peter) has uploaded to Google drive that can be found here https://drive.google.com/folderview?id=0B4azhgT5UI-QWW9TQlhUWE9FWUE&usp=sharing