Been down that road myself. If you haven’t done the following here are some simple suggestions you can do yourself. I had a very difficult time finding a clog in the PCV on my 90 240.
1. As you mentioned, clean out the “nipple” on the intake manifold.
2. Have you replaced both vacuum hoses that run off the flame trap?
3. Is the flame trap screen still there? Consider throwing it away.

#2 was the winner in my case. The vacuum line from the flame trap to the intake manifold was clogged. Very satisfying when I finally found the problem as it also eventually cured a minor rear main seal leak...:)
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Will I buy another Volvo??? We'll see....