Peugeot Boxer Turbo Jerking Whistling

Sounds like a boost leak. But a dodgy boost pressure sensor could also cause similar symptoms. Have you checked the hoses around the turbo and intercooler for any splits or loose connections?

Where exactly should I be looking for the sensor, and what should I be checking for? I'm not really familiar with the turbo system on these.

It's usually on the intake manifold, or somewhere near the turbo itself. Look for a small sensor with a plug going into it. Check the cable and plug connection. If it is corroded or damaged, that's probably your culprit. Otherwise, it might be the sensor itself. Could be worth getting it scanned to confirm.

I checked the plug, and the connection was indeed defective. Replaced the sensor and cable – cost me about 95€. All the symptoms are now gone!