My Mercedes GLC 250 lost all power on a French motorway. The engine did not cut out, but went into limp mode. Mercedes-Benz told me to switch it off then on again. The problem went away, but cropped up again a few days later, at which point I tried a dealer. However, a diagnostic check revealed no fault codes. It was suggested the car could be sensitive to French supermarket fuel, however the problem has also occurred in the UK. Any ideas?