Well to make long story short, I got my new scratch built roabd bike back from the shop that assembled it awhile ago. Long story as to why I didn't build it up myself. I go to ride it and the rear derailluer is behaving wierdly as I pedal and apply force it almost seems to slip and give a clicking noise, it's hard to describe it's not like it's jumping gears but like tthe cage moves forward and then jumps back a little. It happens in all gears. How do I fix this?