My Mitsubishi Outlander has lacked performance on highway driving since new, some days it’s good, some days it’s bad. The dealer checked it at service times and in between, but could not find any problem. He says he would have to experience the problem and go from there.
Now, in the hotter weather sometimes the accelerator pedal goes limp, with very little engine response, so I have to turn the engine on and off again to get a good accelerator response. It may last another 50-100 km until it happens again. The car has done some 19,000 km, so much for Mitsubishi warranty.
I can understand the dealer needing to experience the problem simply to know where to start checking. That said, from what you describe it seems as though the car is going into limp home mode. The fact that you can fix it by turning the ignition off and on again tends to confirm that.
By doing that you are rebooting the computer. It’s possibly got an intermittent failure of an engine sensor or a connection or something similar. Describe what happens with your car to your dealer and insist he investigate and find the problem. That is, after all, what the warranty is all about.