Sadly this sort of fault can be incredibly difficult to find, even by decent diagnostic places.
Someone I knew a few years ago had a Vauxhall Astra which started having electrical problems from when it was nearly new. After many trips to the dealers, and much work having been done (including replacing a lot of the wiring), they scrapped it and supplied an alternative car.
I do hope you can get it sorted though. It's one of those where at some point you may have to cut you losses though.
Someone I knew a few years ago had a Vauxhall Astra which started having electrical problems from when it was nearly new. After many trips to the dealers, and much work having been done (including replacing a lot of the wiring), they scrapped it and supplied an alternative car.
I do hope you can get it sorted though. It's one of those where at some point you may have to cut you losses though.