Look up the fuel pressure gauge issue. Even with a liquid dampened gauge the needle osculates so much you can only get an approximate reading. Also because of the Plastic Lines you cannot easily T into the lines.

Two methods to reduce the needle osculation is a restricted fitting/orifice or someone made a pressure damper similar to one used in plumbing to eliminate water hammer.

If the Fuel Pressure Relief/Overflow valve can be taken apart you can stretch out the spring till it has a free length of 27mm. Inspect the Ball Bearing that is used as the valve for pitting.

Have only 1/2 tank of fuel and dive without the Fuel Fill Cap. If that fixes the issue it is your Fuel Tank Vent system not working well.

Have at least 1/2 tank of fuel and swap the positions of the Fuel Inlet Hose with the Fuel Outlet Hose at the fender well. If that fixes it your Fuel Tank Screen is restricted.

The fuel pressure test might or might not tell if your Lift/Fuel Supply Pump is working well. I say might because it could be either the pump and or the Lift/Fuel Supply Pump.

Air in the system? The old style hand primer?

I have never tried it but if you T into the Fuel Inlet before the Lift Pump there is a vacuum reading in the manual. A low reading would indicate the Lift Pump is not working well an high reading would indicate something is restricting the fuel from the fuel tank. If the vacuum reading gets high fast I would suspect the Fuel Tank Screen.

If you are getting a high vacuum reading slowly increases and you remove the Fuel Fill Cap and the vacuum drops suddenly I would expect a Fuel Tank Vent issue.

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84 300D, 82 Volvo 244Gl Diesel