Sounds like you need to change the Sending Unit.

Also inside of that Tube you put the O-rings on Oil Filter Cap is a Spring loaded Valve. If something holds that Valve open the Oil Pressure will also drop at low rpms.

I don't know how to take it apart to get at that Valve. But, I was able to pull the Tube that is press fitted into the Cap out and used compressed Air to blow down that Tube and past the Valve.

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84 300D, 82 Volvo 244Gl Diesel