In response to Bike Colorado and his problem explained above:

I work for a Giant dealer as a head mechanic and on many NRS bikes I have noticed the problem of "Ghost Shifting" especially when under power. After some trial and error I concluded that the frame's seatstays (where the rear derailleur cables are routed) actually bow slightly when you put the power to the pedals. This bowing lets slack into the cable which causes a temporary misshift. This can be remedied by running a full length housing from the rear stop on the top tube down the seatstay with zip ties and into the rear derailleur. The same may be true on this bike although I do not have any experience with this particular model. Might be worth a shot but I would put my money on that housing loop. I have really liked the rollamagigs on my bikes, but there is no substitute for a fresh cable and housing with a good tune up.

-Ryan