There are many forum threads talking about Drivetrain errors on older models. There can be many reasons: engine, transmission, EGR… From your description it seems to be a transmission problem but if your car goes back to normal after an on/off it might not be a big deal, maybe more of an electronic issue. Make sure you come back with an update after your diagnostic :)

300k km it’s a miracle it’s still running

Absolutely not. They cost way more than the price in the picture in Europe