W123 280CE – Automatic gearbox switches without visible reason

Hello municipality, for a few weeks after about 30 years again a 123 – this time a 280CE . As far as everything is good, but with the autom. transmission is not quite iO. When starting normally softly the gearbox switches into the next gear, but then switches back again – the speed goes then naturally also high – but only for a few seconds and then goes back to the previous gear. If I drive hard at the stop, he drives as he should. I unfortunately do not know whether the car is driving in the first or second gear. After my memory this is a 4-speed gear, or? That brings me to another question: is there a gear indicator, which indicates the currently inserted gear or how to get to the pears in the dashboard? Vllt are the only defective. Can be good after 33 years … There seems to be something like this after 33 years …