I have been trying to think of something that would help.

I would be inclined to check and see if the Glow Plugs are really working or not.

If it is a low compression issue perhaps cause be sticking Piston Rings you might try pluggin in the Block Heater and getting the Block nice and warm before you attempt to start again.

Also the lower amperaged charge for a longer time is better for the Battery.

Also when you are stressing the Starting System it is a good time to make sure the Cable connections are in good shape.

You did not say if you thought the Engine was cranking normally fast?

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84 300D, 82 Volvo 244Gl Diesel