Hi Richard
It may be worth checking all the earth connections from the engine to the body especially the actual connections themselves.
I know a guy who had a similar fault on a rally car (brand new Escort ) . It would start and run for a few mile then just die , it was many events before they discovered the inside of an electrical connection to earth was faulty . The sleeve on the wire was not peeled back enough and was intermittingly making a connection
Hope this may help.
Regards Sean Treacy
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