Have you done any recent work on the car? I'd first check the electrical connection at the coil. Ensure the plug is firmly seated in the socket. Push each wire into the connector...they may look properly seated but looks can be deceiving. Then check again to see if you have spark.

Also check the plug where the white wire connects. The plugs are prone to breaking internally and the wire will still look good extrnally.
The white wire is not getting voltage somewhere.

OK if you used MS's wiring diagram as you said you did I'll assume a few things. The "I" on the start solenoid diagram is not used. Positive feeds the Dura Spark via the white wire when in the start position on the ignition switch. Maybe a loose wire on the 'S' terminal on the solenoid. My best guess would be, and correct me if I'm wrong. The run and start wires should make up at the same time on the ignition switch when in the start position. If the run wire does not make up when in the start position this would act just as you are stating. My guess would be you have a bad ignition switch. Check when you are in the start position on ignition switch, you have +12 vdc on the positive on the coil. Just a thought.