I received my new bike about a month ago and from the beginning I have been having a problem with ****ting the rear gears. It seems whenever I try to shift up from 4 to 5 and all the way to 8th gear, the shifting take some time and makes a grinding sound in the process. Often, when the chain take too long to move, slowing down peddling or stopping peddling for a very quick instance moves the chain up quickly. I took the bike to the store and they did some adjustments and raised the back of the bike with a rope and had me pedal by hand while they shifted the gears and the problem seem to have gone away. Once I took it back and started riding the bike it went back to the same problem. The store keeps telling me it could be my timing, but I really doubt that. Any advice on what is problem and how to fix this it?

My rear derailleur and cassette are Shimano Ultegra and this is a link to the bike:

http://www.trekbikes.com/int/en/bike...ne_4_5_compact