Periodic restricted performance when going downhill

What was once an occurrence every 2 or 3 months now appears to be every 2 or 3 days.

When driving along undulating country roads, my '15 L405 SDV8 is starting to go into limp with the red triangle. It always seems to be after a period of going downhill where the engine is either idling, or at low revs providing a slight bit of engine braking.

The only (relevant*) fault I'm seeing on the iiDTool is an issue about injector 4 data being incomprehensible. However, this error code seems to be there all the time and the milage trigger doesn't match the time restricted performance comes on.

(* it also has a circuit short to ground on the oil level sensor but can't see this being related. I have the thing serviced every 10k mls so as not to let it drop too low)

When not in limp, car drives fine and there's no visible smoke or performance issues.

Clearing the faults on the iiDTool takes it out of limp. It's odd that it doesn't seem to pick up what's triggering the limp though? Any ideas? It has come on about 10 times now and is always when I'm going downhill

I don't have a clue. However, trying to be logical, if the injector is paying fuel when it shouldn't be, possibly that's triggering a fault code on the emissions side when the car is in overrun, ie no fuel should be present so it's trying to protect itself? Just a thought