Is my automatic transmission going out ?

What is your question?

It feels like someone has hit me from behind or like a slipping. I was told it is 3200.00 to 3400.00 to fix. It occurs more each time I drive it and usually in stop and go driving. The mechanic I took it to couldnt make it act up but he told me not worth it to fix..Is this true? Thank You for your time and knowledge