Question about automatic transmission: Mondeo V6

Hello, I’m new here in the forum and have a problem with my Mondeo V6 Kombi, year 1999 125kW, automatic, about 130 000 km. And that is: When I start it all is still normal. After a while the “OD” light starts to flash and the transmission switches much harder. This can happen every five kilometers, or I can drive 20 and more kilometers. Strangely enough, this only happens in the “D” stage. When I turn off/on the ignition the spook is gone again to the next level. I read out the control unit when an error message comes: “Converter bridge coupling does not close”. But I can’t quite understand that, because I should have already heated the converter in case of a defect of the coupling. Oil level of the gearbox is ok, the oil has a pink color without impurities. Who knows advice? Gruss Richard