I’m amazed that you managed to find such a convoluted reset procedure, even if it only provides a temporary fix. You may well be right that this behaviour is linked to heat build-up, especially if it happens after several hours of driving.

It’s interesting that you mentioned the engine sometimes goes into limp home mode, because the transmission’s behaviour sounds a lot like it’s going into limp home mode, too. Limiting the gears the vehicle will select is a classic symptom of limp-home mode, in fact.

The real question then becomes, what’s causing it to do so. Limp home mode can be initiated any time the vehicle thinks that continuing to drive at full power or speed will do further mechanical harm. As such, there’s a huge range of things that will cause limp home mode to engage, including a leak somewhere in the turbocharger’s plumbing, low coolant level, low oil pressure, low fuel pressure, a faulty sensor and many more.

So the first job is to scan the vehicle and see what fault codes are thrown up by the computer. From there, you can home in on the actual problem and fix it. But without those codes, you’re kind of flying blind.