Thinking of selling my Duster

Well I went for a nice cruise in my car today. The weather was really nice and the car ran good for the most part. On the way home I noticed that when the car shifts from low 2 to drive there seems to be some slight slippage or weak shifting in the transmission. I just bought this transmission last year and had it completely checked over and was assured it was fine. At this point I am wondering if it would just be better to sell my car and either look for another car or get out of the classic car gig alltogether. I am really frustrated. I have literally spent thousands of dollars on this vehicle and I just do not know if I want to keep spending. I know I'll never get back what I have put into my car but was just wondering if it's a good time to sell. I have had this car for 11 years.