Start Up Probs.

On this 1985 300TD, when the key is turned to start the car (glow plug light having cycled) on occassion there is NO result. There is no click from the starter no fade of indicator lights - total flat silence and no start. On the very next attempt (key off - key on) the car starts perfectly and normally with no sign of difficulty or struggle. It sounds to me as if there is an electrical disconnect somewhere. I have a multimeter but need some help knowing what to test and in what order. Where should I start and how should I proceed? Thanks.