Technical ducato 2.8 jtd elec problem

[gary demo man]

hi we have got a bessacar e495 motorhome on a ducato 2.8 jtd, the problem is that when we start off on our travels all lights work then after about 20-30 minutes the dipped beam, hazard lights and the cab lights go off, then after the engine has cooled they all work again, can anybody help thanks.