Citroen C5 X7 3.0 HDi intermittent no start

[dieselmanfcf]

Hi All,
I have a 2010 Citroen C5 X7 3.0 HDi that quite regularly won't crank.
It usually displays the following faults:
Gearbox Faulty
ABS Faulty
Parking Brake Faulty

When I connect it to my Chinese Diagbox, it usually has a communication error with the Engine ECU, but sometimes it can't communicate with other ECU's too.

If the car starts, it will happily run for hundreds of kilometres with no issues, but if I stop the engine, quite often the errors appear when the ignition is switched on and it won't crank.

If I leave it, it eventually will correct itself, but sometimes it has to be left overnight before it will again start.

I have replaced the usual suspects, brake switch and BSM (engine fuse box) to no avail.

Sometimes when the errors occur, one of the relays in the BSM makes a repetitive rhythmical click click-click, click click-click etc.

Any suggestions?

Thanks.
Leon

Australia.

[Paul-R]

How old is the battery?

[dieselmanfcf]

The battery is new. 750CCA