In addition to what the others told you so far, with the engine off, check the air plate for sticking. Since you have cleaned it, probably it will not stick.

Then pull the injectors, have new seals on hand, and install them for kicks.. Flip the key to on and lift the air plate as you watch the injectors spray.

They should resemble one another in spray pattern. If not clean them the best you can. Varsol is what the dealer uses and in a pressure pump to blow them out with the varsol.. I use oven cleaner as I do not have the Volvo tooling for this.

I am not sure what down under cars have, so if you happen to have a vacuum pod on the distributor check that. Pull the hose from the manifold end and clean it well enough to draw (suck) on the line.... See if you can stick your tongue to the line and the line holds it there... If not that will be a hesitation for sure.

Do you have a product known in the USA as WD-40? This is supposed to be a light lube spray, and it is only so-so for the claims, but it makes a great tool for seeking vac leaks in general.

With that or something like that, and some folks use a un-lit propane torch the same way.... You can feed a fuel rich air mix around any and all vac lines listening for any RPM change.

If no leaks are found in the engine compartment, then seeking leaks in the dash might be in order. Any vac tubing has 2 ends no?

I hope this helps.. Mac