Slooow automatic transmission engagement --- normal ?

I have Renault Megane 1.6 -97 with automatic transmission. During the latest months the transmission have had a delayed engagement when I start after a couple of hours pause (or in the morning). The delay is usually some tens of seconds, but very often I have to wait minutes. Normally I don`t wait so long, but press the accelerator until the car suddenly rushes shortly forward. After that the transmission works quite well. The delay is longer when the temperature is over 15 degrees Celsius, but in colder temperatures the transmission works OK. Last week the car was in Renault car service. They changed the transmission oil, but they did not recognize anything. Maybe because the car did not have long enough pause. They did either say nothing to my questions. Perhaps because they are not used to automatic transmissions here in Finland.
Has anyone had similar symptoms in the transmission? Can someone, please, explain what`s the problem. Can a professional fix it in a reasonable working time (< 8 hrs).
Many thanks in advance. Regards

internal rubber seals have hardened and shrunk. The seals will seal better when the fluid is cold and thicker but when the fluid warms up and thins, the seals leak excessively. There are fluid additives that will often soften and rejuvenate the seals. The other alternative is a complete overhaul. John