follow the injector rail wiring loom, you should see an 8 pin multi plug clipped to a metal plate on the inlet manifold. Wiring as such doesn't tend to play up for no reason, especially things like this, usually the more powerful items if any. So your looking for all the weak points in the chain, so as pollocks said, the back of the clocks, and this wiring multi plug which after the years get damp and corrode etc.