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.

There have, indeed, been issues with this transmission with Mazda authorising a service bulletin to attend to some vehicles. However, it‘s generally thought that affected vehicles were built between 2011 and 2014, after which point, the problems were fixed. Which is not to say your 2015-built car doesn’t have the same problems.

The best way to proceed is to ignore the dealerships and go straight to Mazda Australia’s customer service department. Unfortunately, your car is now 10-years-old and well and truly outside its warranty, but I agree that 120,000km is not an acceptable lifespan for a component such as a modern automatic transmission to fail.

You can probably rule out anything like a split turbo hose as this would cause the vehicle to lose power all the time, not just after prolonged use. Modern turbo-diesels like the one in your car use a range of electronic sensors and controls to keep everything running properly. It could easily be that a sensor is sending erroneous messages to the computer. An electronic scan of the vehicle should offer some answers.

The other possibility is that the fuel system is not keeping up with the engine over time. This could be due to a blocked filter, a blocked fuel line or return line or even a fuel pump overheating. A check of the fuel system would also be in order.