Had you done an work under the hood lately?

Spray down all of the Linkages ball areas with WD-40. It is not a great lube but if the sticking goes away you know where the issue is.

Inspect all of the under that hood linkages with special attention to the pivot on the fire wall.

There is a throttle return spring under the Fuel Injection Pump and maybe another one near the fire wall (memory is not clear on that one).

Pull out your mats and inspect all of the pedal linkages inside of the car and where the pedal bolts down to the floor.

I don't know why it would suddenly cause an issue but if your Rack Damper/Governor Idle Pin is in to far it is said it can run away like that (the idel speed is not supposed to be adjusted with that part). Back it out and if that issue goes away that is the problem.
If that turns out to be the issue use the nrmal idle adjusment screw on the Engine side of the Fuel Injection Pump and then slowly turn in the Rack Damper/Governor Idle Pin till you have removed the idle shaking without increasing the idle speed.

Take a picture of the Cruise Control linkages and disconnect them from the Throttle linkages and wire them out of the way to see if that is the issue.

The only other way I can think of that your Engine could get something to run on is if your Turbo compressor side was leaking oil into the Intake or a whole lot of oil would some how have to go by your intake valves and down into the cylinder.

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84 300D, 82 Volvo 244Gl Diesel