1996 C280 Auto

Good day
I would appreciate any advice here. I have a 96 C280 and when the automatic box shifts from first to second it does so with a loud and
hard "thunk". It is extremely severe under hard acceleration. If you can get it to change at low engine revs it is quite smooth. I have had the GB serviced by having the filter and oil changed and also improved a little when I put a bit more oil in the box. How easy will be to solve this problem?
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