As I mentioned a few weeks back, my brother has a V50 2.0D which is giving him some issues.
I had a look over the weekend. Unfortunately, it's been "Motor Traded" to the tune of about £2k before I got a look in, so no doubt that has caused some issues which need to be corrected before the real diagnosis begins.

Symptoms are that it "surges" a bit under light load around the "normal" driving speed range (1.5k - 2.5k RPM). Often it will slip into limp-home with very poor performance (i.e. no boost) until restarted, whereby it recovers.. for a while.
It starts and idles perfectly fine. On full-bore acceleration the performance is probably about right and it pulls smoothly. No nasty noises, smells or smoke over and above the usual.
On taking a look at it, I discovered the following:
Intake air temperature sensor trouble code, and reads constant-30 Deg. C on live data. This is the post-turbo sensor in the intake manifold. Looks like a little bead thermistor that's completely open circuit. Would have been eliminated by now if the muppets had sold me the right part.

Apart from the above, ECU is happy.
Couple of connectors left off, Breather pipe disconnected from crankcase, some signs of bodging of the wiring around the DPF pressure sensor, which looks new. Yep, it's been in the hands of some muppets. None of this appears to be causing any issues, and didn't help once corrected.
Oil level is about 20-25mm above the max mark on the dipstick. I suspect some fuel has got in there!

I can imagine the limp-home issues might be to do with the temperature sensor, in that the ECU gets nervous when it's been on boost for a little while without induction temperature feedback and stops play, but it really doesn't seem to be the whole story.
I can't help thinking the surging under light load might be a fuel or boost pressure control issue. I've heard that fuel pump issues on these can cause the oil in the sump problem, and might cause fuel pressure fluctuations. On the other hand, fuel in the sump has also been blamed on failed DPF regenerations, and the latter has, apparently, been recently replaced (although god knows why they didn't sort out the oil level issue).
Surging manifests itself as a regular about 1 Hz cycling of torque output. If you hold it at about 2.5 K RPM fast idle, you can also see and hear the revs fluctuating +/- 200 RPM or so about once a second. You can hear the turbo whine cycling up and down in pitch a little at this rate too. Sticky wastegate / vane actuator perhaps?

IAT sensor will be replaced ASAP, along with an oil change and correction of level. Any further ideas?