I tried the cardboard trick without any luck - the machine still died with cardboard in between the contacts on the 10's relay.  (Single piece of cardboard tried at various contacts, not all at once).

The machine seemed to die when a drop target was downed - cleaned all of those contacts as well as the plungers for the coils and still no better.  The machine has gone back into freeze mode, no flippers but the ball kicks out and no scoring at all.  I don't know whether it has got something to do with heat - as the machine warms up the fault seems to happen more often and then gets to a point where it just doesn't want to play anymore. 
This machine is well and truly doing my head in at the moment.