Won't always start

[nazrat]

My 1985 300TD almost always starts after a normal glow plug cycle. Sometimes it just cranks though and doesn't fire. When that happens it is immediately obvious because this engine always starts after very few revolutions. When it passes the Xth revolution (say 5 although it is hard to count) I know that it isn't going to start. If I turn the key off and immetiately back on to start (no glow plug wait) it fires right up. Could this be a sticky vacuum shut-off on the ignition lock? Maybe it's the shut-off on the injector pump sticking?

Who's seen this before.

-Tad