had exactly the same problem on mine. and it was the relay.
It started happening every week or two for a few minutes but got more and more frequent.
I saw the price in the dealer and cried. Then I took the relay out (in the panel to the right of the steering wheel - it just pops out), took it apart, cleaned it and put it back. I then realised that I had just cleaned the boot release relay.
Then I took out the right one, cleaned it and put it back. That was 5 months ago and it has been fine since.
156 2.4jtd working it's way towards perfect condition. Loving the diesel warpage!