Bought a brand new Bianchi Vigorelli with full Ultegra setup. Has my LBS put it all together and out I went. At various times in my ride it feels like my chain is going to come off, when I look down the back deraillieur seems to be twitching like it is going to shift. The bike simply skips, but it catches itself and I am fine for another few strokes and it does it again. I will shift through all the gears to get it to stop doing it, but it does it again after a few minutes. I know nothing about bikes, is this as simple as tightening a screw.