[trumpetdropper]

It might be worth checking the oxygen (O2) sensors if its running rich as indicated this will potentially choke the fuelling making it chug at low revs, but fly on full throttle.
As with all things Volvo, buying good branded or O/E is far better in the long run.
(my old MR2 used to eat O2 sensors and the cheap ones wouldn't last a week)
Sometimes you can clean up the old ones, take them out carefully as they don't like to be knocked about and spray them with carb or brake cleaner and when dried out refit with a little copper grease.