Originally Posted by
Al1943
This problem (auto-shifting) is usually caused by the shift cable stretching. But in your case it may be the fixing bolt that holds the cable to the derailleur is slipping. If you don't know how to fix it take it back to your dealer. Early adjustments on a new bike should be done at no cost to the buyer.
Al
High quality shift cables do not stretch. More than likely the cable system was not pre-stressed after initial assembly. The symptoms you attribute to cable stretch is actually the ferrules seating into their housings fully and into their stops.
1) The OP needs to fully stress the system, one way to do this is to hold the derailleur and force shift it with a reasonable amount of force to seat everything.
2) Once that is eliminated, you can look to a bent derailleur hanger as a cause. It is very common for new bikes with AL hangers to be bent out of the box.
3) Let's make sure the OP isn't skipping, or is using incomplete instructions to adjust the derailleur
http://www.parktool.com/repair/readhowto.asp?id=64
4) The final thing would be to check incorrect cable/housing system, whether from bad housing cuts or excess friction due to any number of causes.