I put together a commuter with used parts. The wheel set came with a 7 speed cassete.
Now that I have everything together, when I peddle the bike the gears slip. Could I have the master link to tight or are the casset paws worn? It is not a mild skip but one that feels like it is skipping a few teeth at a time. Thanks to all that respond............

New cassette, and a new chain is a good start .

Thanks for the response. I figuered out what the problem was. I forgot to tighten the rear axle bolt. It works fine now and luckily the wheel didn't fall off.
Thanks again........