I have replaced the chain, freewheel, rear derailer, and crank. Now the chain skips when pushing big gears under pressure. I've been doing sprints starting in the biggest gear at 10 mph and hitting 30, it's very bad when I do this. I don't understand why it would do this with all new parts. I had this same problem with a different bike some years back after I streched a chain badly and had to replace everything.