Based on the above figures, I would suspect the main earth lead from the engine to the chassis.
It's made up of many copper strands and some can break internally near the connectors causing voltage drop.
If you have a know good jump lead, you could connect that to the engine block and chassis instead as a bypass to see if that improves matters.
The earth strap is also the main conduit for return current for other engine components so could realistically cause running issues.
Thank you too btw for your kind gesture.
much appreciated.