Road Wheels & Road Wheel Recommendations?

This sounds like the wheels moving in the drop-out. Have you considered swapping to better skewers (shimano have the best design).

Difficult to imagine the wheel moving so much near the axle through flex alone. You can also space the rear cassette out up to 1mm. Which could help. Probably better to solve the problem, and not just the sympton though.

I'm pretty sure it is the wheel flexing. The cage is very close to the spokes in the top gear and not helped by being a short cage. It's also probably in part down to my crap climbing techniques rocking the frame from side to side. I shall pay more attention next time I ride the bike although it's currently in hybernation.