1996 Cadillac Seville 4t80-e

I am getting ready to purchase the above car, the dealer used as a personal vehicle. He had the trans. Serviced before going on a trip, the trans worked fine, he started having problems with overdrive after getting to his destination. When the car reached operation temp the overdrive would surge. I was told that (by him that the tranny needs to be replaced) I dont buy that I think it is either a solenoid or a pcm problem since it only acts up a operating temp. What are your views on this matter?