This is occurring on my Avanti Discovery 8, which is a hybrid bike with an in-hub 8 speed.
Problem is I seem to be applying so much torque it's pulling the rear wheel forward in the frame, loosening the chain. I've tightened the rear nuts as hard as I can - to the point where the nuts got a little damaged (hex edges started to 'flatten' inside the spanner) - but the wheel keeps moving forward as soon as I go up a hill and apply pressure. You can even see the scrape marks where the nut and washer has been digging into the frame and being pulled forward.
Is there any solution to stop this?