i took my rear wheel to shop yesterday to get some spokes replaced. go back today and they just gave me a new wheel instead. come home put it back together. pedal it on the stand to go thru the gears and now the chain will not even stay on the smallest cog it just jumps back and forth between the last 2 cogs then im watching the cassette and it like floating moving around a little. thats not supposed to happen right? how do i fix this?