On the surface it sounds like the Ignition Switch and or if it was my year and model the Steering Column Lock that is between the Lock Tumbler and Ignition Switch. Don't know what setup yours has.

Don't know if it is possible for the lock tumbler to do what you describe.

How to get some idea what the issue is. Use the volt meter on the Battery and see what happens when the key is turned to the GP position and see the voltage. If there is not a voltage drop your GP relay may not be getting voltage to work.
Do the same volt meter test on the glow plug relay connector to see if it is getting voltage.
When my GP relay strip fuse cracked for a bit, I got intermittent glow plugs.

You can do the same with the wire going from the Ignition, neutral safety switch to the starter solenoid and see if it is getting voltage. No voltage at the Starter can mean the Ignition Switch and or the Neutral Safety Switch.

If you got under the Car and disconnected the neutral safety switch and identified the terminal that supplies the voltage and probe that to ground through you voltmeter, I think you could tell if it is the ignition switch/steering column lock.

Sadly, things are often a system of components and any one of the components or more can bring down the system. Sometimes you have to trace the electrical circuit back from the problem to find out where the next component that could be a problem is.

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84 300D, 82 Volvo 244Gl Diesel