On a TDI engine? Scanned a passat this morning with this issue. Not a regular occurrence. Would often pull hard on the motorways/flat, but when asked to go up a hill off a roundabout, would throw into limp mode.

I was thinking either boost leak or actuator vacuum hose. Could it be anything else?

Sticky vanes on the turbo by the sound of it

I thought that, but that usually causes overboost no? Not negative deviation where its not making the requested boost?

Not sure to be honest, the symptoms you describe is the same as sticky vanes. Try a search in either the octy or superb sections there's a simple way of testing it by moving something lol

Mine went into limp mode on the motorway once, pulled over at the next services and scanned it, and it was negative deviation. I never managed to get it to happen again, but about a month afterwards the turbo cried at me on track at Castle Combe, with later inspection the vanes were fubar'd. I'd be looking at sticky vanes also in this instance.

Probably depends how the vanes have stuck. If they've stuck "open" then nailing it will get you overboost (positive deviation), because the vacuum controller can't close the vanes and stop the turbo spinning faster and harder. If they've stuck "closed" then you'll get negative deviation as the ECU will be trying to work the turbo harder to accelerate up a hill than it would need to on the flat, and with the vanes stuck "closed" then it can't adjust to get the turbo spinning properly. Obviously I don't mean literally open/closed, but the two extremes of VNT operation.

Either turbo for the valid reasons mentioned above or a split in the VNT control line are usual suspects.