I would think, bad connection somewhere, which would not show up on diagnostics. problem is finding it, you would need to start moving wires about, when it does it, to slowly narrow the area down, as click under dash, thats probably the relay for the glow plugs, check around that area first, give the wires a slight pull, if one comes off, could be that, as a slight pull showed not pull it free. make sure you can see where the wire connects to in case it does come off, you will then be able to refit it, without calling for a garage to do it. most spade terminals can be tightened by gently squezzing them together, to get a tight fit
SD
November 2011