Why not check the Vacuum with a Vacuum Gauge. I beleive you can get them as a free rental from places like Autozone. They are usually a combination of Vacuum and Fuel Pump Gauge in one gauge.

When I had a similar symptom it turned out the Check Valve the srews into the Vacuum Pump was all gunked up inside. Sprayed it out with WD-40 and everything went back to normal. The Check Valve is where the main line screws into the Vacuum Pump.

Some people have removed the Vacuum Pump Check Valve and found all of the parts had fallen out of it. Also don't use brake cleaner on it as it has plastic parts inside and brake cleaner messes with some plastics.

Also it does not cost anything to take a look at the Check Valve.

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84 300D, 82 Volvo 244Gl Diesel