You might be looking at two distinct issues here. The high idle could be caused by any number of things including a dirty intake system, or something electronic. It could simply be that the idle was set too high the last time the vehicle was serviced.

This may be contributing to the feeling of the engine shuddering when you switch it off, but in reality, a lot of diesel engines exhibit this. It’s all to do with the heavy-duty nature of the diesel engine design. Because of the high combustion chamber pressures inherent in the diesel’s design, the whole thing needs to be more physically robust than a petrol engine of the same size and capacity. That means everything from the crankshaft to the con-rods, pistons and even the cylinder head and crankcase need to be made stronger. And that means more material and that means more mass.

This is why diesels generally don’t rev as high as petrol engines (there’s much more reciprocating mass) and why they can feel a bit lumpy when you load them up and even at idle. So, when you switch one off, it can feel like it’s trying to jump out of the engine bay. However, if this sensation has become more pronounced over the years, it might be wise to check the engine and transmission mounts for wear. Any wear or looseness here can contribute massively to vibrations from the driveline being passed through to the cabin.

I’d start by talking to a transmission specialist. There’s a fair chance they will have seen these exact symptoms before in this make and model and will know what’s wrong without wasting any time or money on guesswork. It’s probably also worth going back to basics and checking the transmission fluid level, as low fluid can cause all sorts of drive problems. If, on the other hand, the gearbox is worn out internally, you’ll either need to rebuild or replace it, but don’t rule out something less expensive like an internal selector fault or even a torque converter problem.

This sounds more like a gearbox that isn’t selecting fifth gear properly, rather than one where it’s leaping out of fifth. For it to pop out of gear when cruising along suggests it’s not getting into gear fully in the first place. You might be able to adjust the shifter’s position and alignment and therefore get fifth gear to engage more fully.

There’s also a chance that this refusal to select fifth gear is the result of bent or worn selector forks within the transmission itself, and that’s a gearbox-out job unfortunately. So the first step is to get underneath with a torch while somebody inside attempts to select fifth gear. If the selector linkage runs out of range of motion, you might just have an adjustment problem which can be pretty easily sorted.