2000 Volvo S60 2.4T SE Geartronic with 72,000 miles.

My car is feeling a bit sluggish... It's not slow, but theres no kick in the back when you floor the throttle, like there used to be. It feels like the turbo is either not working, or working at reduced performance.
Other symptoms include:
... At high revs (over 4000 - especially noticeable in 2nd gear at around 50mph), if you have it in 2nd gear and you put your foot down it will just shudder for 2 or 3 seconds with no acceleration whatsoever before accelerating as normal. This happens 4 times out of 5. The weird thing is, if you are in auto mode so that it's in a higher gear at 50mph, and then floor it so that it kicks down to 2nd, it will just accelerate as normal without shuddering.
... There is sometimes a slight whistle as you accelerate (usually only in first gear but sometimes I hear it in second as well) from the engine, which sounds like the turbo spinning. It is not loud, and this may just be a normal feature of this engine.
... The "Emission System Service Required" message comes on the computer. If I reset it (by pulling out the ECU fuse) it will stay off for several days and then randomly come back.

I have tried injector cleaner, and have installed a new boost control solenoid. Neither of these made any difference.

Another hint is that the first time I noticed this was after the car spent 2 weeks in the South of France in hot temperatures, and it first appeared on the hottest day.

Ideas?