This sounds like a classic case of worn synchromesh rings inside the gearbox. These rings are designed to make smooth, easy shifts between gears but as they wear (or even break) they lose that ability and the gears become difficult to change. It’s also common for this problem to be worse on downshifts compared with upshifts which is why you can shift all the way up to fifth, but have trouble coming back down through the gears.

My money is on third-gear synchromesh being your problem. By shifting from fourth back to second and then third, you’re effectively `borrowing’ the second-gear synchromesh to soften the shift into third.

Unfortunately, the fix is a gearbox rebuild. But before that, it would be worth changing the oil in the gearbox and checking out a gearbox additive that helps with shifting feel and smoothness. If that makes a difference, great, but you may still ultimately be looking at a new or rebuilt transmission.

The other thing to know is whether the gearbox is reluctant to select reverse gear with the engine running. If it is, you might instead be dealing with a worn clutch which is much cheaper to replace.

The sound of air hissing on a turbocharged engine is often down to a leak in the intake trunking that takes air from the turbocharger, through the intercooler and into the engine. As this air is under pressure (from the turbo) you’re likely to hear it above the rest of the engine noise. If it’s only a small leak, you might not notice the fall-off in performance (or boost as you’ve described it) but it will only get worse as the damaged part of the trunking deteriorates further. Eventually, you’ll start to feel the leak’s effect as it allows more and more air to escape rather than being forced into the engine.

Checking the boost pressure with an accurate gauge will tell you if any boost is going AWOL, as can an electronic scan which might throw up a low-boost fault-code. But if you can hear the noise and have found the leak, replacing that section of plumbing is the logical first step. If that makes the noise go away, you’ve solved the issue.

Sometimes the leak is down to a split in the trunking, other times it can be as simple as a clamp that has been loosened by the constant pressurisation of the system every time the turbo boosts up. But it needs to be fixed as it can throw the air-fuel mixture way off target and lead to internal engine damage. If you can’t find the leak but suspect that there is one, a specialist workshop can perform what’s called an EVAP smoke test where the intake tract is filled with smoke. At the point where the (visible) smoke escapes, there’s your leak.

You’re going to be hard-pressed to show that the problem was either a design or manufacturing problem, as it’s unlikely to be either. Even though the wiring loom just sits and doesn’t appear to be subjected to any stress it being subjected to all the bumps, vibrations and whatever else is happening through the vehicle in general. It could be a faulty connection; it could be any number of things. Pay up and get on with your retirement.