I have a vintage Trek Elance that has a problem and I am stumped. When I am climbing a hill in a low gear standing up, the chain slips. I bought a new Shimano 6 speed freewheel cassette and my chain has very few miles on it. It was doing the same thing with the old original gears about a year ago so I bought the new chain. When it continued to slip I just hung it in my garage and rode my other bikes. I recently bought new handle bars, brakes, grips and the cassette and was excited to ride it again. Does anyone know what this could possibly be? I am about 225 pounds but it has never happened to my other bikes. Thanks, I'll post pictures of the bike after I make them small enough for this site.