Nick,
Your problem sounds very similar to the one I had recently. Very hard starting, rough idle, low rpms.
I and my local mechanic and volvo dealer worked on ths this for about a week and couldn't pin point a cause.
Just on a whim or brain fart whatever it was, I went to a local u-pullit junck yard and found a lower mileage one that mine and got fuel rails, injectors, relays and anything with the feul system I could.
I started changing things out that seemed the most unlikely since nothing seemed to make since. That was the cold start injector on the bottom of the intake. Lo and behold the old girl fired right up, ran as sweet as she ever had. the mechanics and I just stood there and scratched our heads and said some things that we normally wouldn't but it is still running as sweet as the day she was born.

Good luck
Tony

By the way that cold start injector is a real PIA to change.