On my BTwin, following a recent cassette & chain replacement, the chain is slipping on the cassette when put under load (so, for example, if I'm in too high a gear pulling away from a junction). It's as if the chain is trying to rip itself off. LBS have adjusted grearing, cable tension etc., to no avail. He reckons it's a freehub issue, which would be annoying as the wheels and hubs (Shimano R501) don't have that many miles in them. Would a freehub body replacement do the trick, or is it a new wheel for me?