Not sure to be honest! Problem is, it's not just one system that's faulting but it does seem to always be a system that takes an engine speed signal. Whether said signals are routed through the BECM or not I'm not sure.
Might be one
@Rick-the-Pick or
@martyuk may know.
Still think you'd be better off if you could do some checks with a nanocom to be honest.
Sorry, only just seen the tag in this now..
I doubt that it's the BECM at this stage - mostly because if your engine speed is showing 64 in the engine ECU aswell as autobox ECU, then chances are it's being derived from the engine ECU.
Crank sensor, or No 4 injector wiring could be culprits as to engine speed, but either of them failing shouldn't cause the dash to drop out or other spurious faults. I had a diesel where the No 4 injector wiring was faulty and the vehicle would still start and the rev counter would work. It was gutless and didn't like revving, but it didn't lose the tacho.
It can't hurt to try swapping the crank sensor - they are probably the most important one - so even if it doesn't fix it, it's well worth having replaced it. They are a hall effect sensor, and even if they look ok, can get a break internally.. (Quite known for it on the petrol models where they won't start when hot, but OK when cooler ).
My first thought on this, if it's not the alternator is to check the exciter wire from the alternator to the BECM. This tells the BECM that the Alternator is running and putting out charge voltage. If this drops out, then I have heard of various strange things happening.
In the alternator, it's a Brown/Yellow wire, and goes through pin 25 of the connector under the coolant tank (that looks good on your one).. to pin 15 of C1289 on the BECM (20 way Green connector on the front edge of the BECM).
I have seen a couple of times that the ring terminal can corrode where the wiring is crimped into it (including on my own P38), and if this is the case, it could be causing an intermittent connection. I'm not sure what the BECM's response would be if it thinks there's no alternator charge when the vehicle is moving, but Alternator Fault (and probably gearbox fault) are what I'd expect to see.
If that wire checks out, then as someone else mentioned - I'd be trying to check the alternator output carefully when the vehicle is running, as to me, if there were an engine issue with something like the No4 injector or crank sensor, then I would expect there to be engine performance issues aswell.
Hope this helps,
Marty