poorly 3.9 efi any thoughts?

Did u manage to get to the bottom of this? Was it the injectors??

Have the same problem with a Disco 2 after head work.
As I work abroad,have plenty of time to troubleshoot on-line in my off time!
After checking the obvious (which you apparently have), it seems that a faulty throttle potentiometer & sundry other engine & road speed sensors that all feed info to the ECR can give these symptoms.
Maybe the time has come to bite the bullet & get your local LR specialist to put a digital analyser on it!

Recently had an injector fault

Got a misfire during a weekend away

Fairly quickly located the moss using water on headers method, and established not a spark problem.

Without a multimeter couldn't tell for sure but guessed at bad injector and sure enough it was, swapped injector wiring around and still same so I had to guess in the field.

I got a few spare when I got home and now carry spares in the car.

It had died completely, no continuity through it, and it was simply there one minute and next or stopped working.

check both temps sensors (coolant and fuel)