Hi Martin. You say that the car runs ok for about 4 day's. ?? Does that mean that something goes wrong after that or thats as long as you have tried it for. ??
According to the Diagram, pin 2 goes directly to Earth, So bridging an earth in should cure it. ?
Has it got one of those big round connector plugs where the engine harness plug in, ?? if so I would check that, Otherwise i would be tracing the brown/white wire from pin 2 to earth. Or if it does run ok, leave it with your earth bridged in. ?
I think you will be causing yourself too much work by removing the instruments etc.