I had the same issue on the used Santana I bought a couple weeks ago. After changing all cables and housings it is working perfectly now.
Even if you only have 1000 miles on the current cables, if water or mud happened to enter the housing your cables may not moved freely at the housing ends. Another issue could be a too short housing that bends the cables too much when turning or when transporting the bike. You could also have dirt in the derailleur mechanism or a weakened spring.