you can check if the frame dropout is bent, I think they can easily get bent....
brumos said:Check your derailer hanger.
I had this problem right out of the shop. Took it back, it was slightly bent so they straightened it. Rode it home and it was still ghost shifting. Went back to the shop and they put on a new derailer. Worked fine for a a couple of rides then it started ghost shifting again and in the process bent a few links of the chain. Finally got them to put on a new hanger (no bottle opener on this one) and a new chain.
It's been running smooth since.