troubles with automatic gear with opel Omega 1993 2.0 l?

I have a problem with an Opel Omega 1993 2.0 l,
The car drives no prblem,just ones in a while the S light on the automatic gear appears, and the car goes in protection and it doesnt go any more, (it goes very slow for a good while),
I ve changed the gear oil and filter, but it didnt change much. Would you be able to help me?
Thank you very much and have a good day,

ANGELO....Sardinia

Angelo sardinia