Like most of you, I have several bikes. One bike has me confused. It's a 58cm Specialized Sequoia Elite (2008) and the issue is that whenever I stand while pedaling up a hill. the back wheel loses traction. It feels like the whole back end is twisting. This doesn't happen with my other bikes which include a Large Giant Escape, a large mountain bike and a 56 cm cross bike. This problem has never occurred on any previous bikes I've owned. So my question is, what's up with this one?