I got my 95 850 with close to 200K on it and found the hoses, PCV controller,
and the intake manifold clogged with a hard crusty mess. I pulled the intake
manifold and found that the plugged part was at the PCV contoller, hoses and intake only. The oil separator was clear.
I've wondered since if you could just remove the controller and hoses, and
clear any intake manifold port blockage via the opening at the throttle body
while leaving the intake manifold in place. I got the harbor Freight CRT boroscope for short money and think I;d first try to clear that little plugged port from inside. Probably you could bend up some drill rod or whatever to make
a suitable tool and then clear out any debris with a shop vac afterwards.
Thats where it might be handy to have that boroscope to see what's going on.

Another clue that I got from my local Volvo garage guru is that you should switch over to Synthetic oil since it doesn't form deposits so fast if at all.
The downside is maybe some seals might start weeping for a while so maybe
when switching to synthetic it would be smart to use a real seal treatment with it.
I did here and it seemed to work, but after maybe 1000 miles instead of the 500
that they claim.

If you do yank the intake manifold, I'd urge you to get a set of stubby swivel
sockets (3/8 drive) They make that job easier. Change out your old vacuum hoses and elbows, since they are old and untrustworthy and waiting to stab you in the
back without any warning.

Post back with any good news, Bill