L series no start

Hello ,I have a 1997 L series with 166k on the clock most things still work on it,but sometimes it turns over and does not start and has to be left for a hour or so . It will then work ok for the next couple weeks .When this happens the injection pump is buzzing away but its like the fuel stop is off. Runs ok and has no fuel issues when running. Could it be immobilisor or has anybody had anything similair.