Inlet manifold gasket?

Gavin,

I've recently had the same problem myself on my 2 Litre B200E engine (same
year).
It turned out to be fuel injector seals around engine block, they are like
"O" rings.

To make sure it is this, with engine running spray some WD40 around the
bottom of the injectors and listen if the engine changes note, if it does
then seal has gone and your sucking in air, causing tick over problems.

There is an inner & outer seal for each injector, only the outer ones on
mine were gone. Very easy to replace, just undo one bolt, pull out the
injector, pull off seal, then replace with new one, refit injector, re
tighten bolt. That's it.

Cost about ?1.50 each from a Volvo dealer.
I had replaced everything you had, spent a fortune (like you) then a
mechanic had a look for me and found these seals had gone.

Good luck.
Richard

Did you clean out the throttle body?  Maybe overdue for FI service?
Maybe you can get your car to run ok at a lower idle, so that the low
idle isn't really a problem.  I had to do that on a 740.