Surging rpm when coating down hills - auto trans


this is my third post today - i am full of prolems. I pick up my new car at the dealer tomorrow (none too soon) but this is about one I’m selling:

'89 Cary, auto, runs great, EXCEPT: When coasting down long (but not particularly steep) hills, the engine sometimes surges about 1000 rpm several times, so the ar feels all jerky. Could this be a problem that would end up getting worse for the next owner?