Something is grabbing at the derailler and yanking it. This could be caused by the chain. It could also be caused by an incorrect derailleur alignment; the rear derailleurs pulleys could be making contact with the cogs thus that grabbing effect, if this is the case then you need to put the cage into more of a downward angle so that when the derailleur is in the middle gear the cage should be vertical. This allow the cage to move either forward or backwards when shifting to bigger or smaller gears to take up the chain slack.
The only other possiblity, but remote, is that the cage itself is too short for wide spread or range gears (assuming your not using racing gears) when a long cage would have worked better. I can't see an LBS making that kind of blunder...but? The short cage would allow the cage/pulleys to contact with the gears especially in lower and higher but not in the middle if they adjusted it for the middle.