[SwedishBus]

The bottom pipe under the engine, running compressed air from the compressor to the intercooler inlet stub, has a couple of stub hoses that are permanently wet with oil, and therefore rot over time. If you cannot find anything up top, or smell anything, it could well be the connections at the bottom.

Failing that, you will need a careful listen around the intercooler itself to hear for any air leaks. Most garages have a leak-test machine of some description or another; I have a "Smoke Pro" machine that emits lots of smoke for this sort of leak detection. It might pay you to borrow the services of a local garage if you are struggling, as they can often find this sort of thing in minutes.

Don't rule-out that the boost actuator might be sticking or incorrectly plumbed, as there is usually a reason for the intercooler circuit to fail......