Hi,

I have a Mountain Bike with 8 gear shift and 2 chain rings. The gears seem to change smoothly when riding casually however when I apply consistent pressure on the pedals to pick up speed whilst on the highest gear, the chain seems to be in contention. It feels as though the chain is not in gear properly e.g. the chain shifts due to the force applied to the pedals and makes a ' tuck ' noise. When I feel the chain has finally gone into gear and apply pressure to the pedal again, chain shifts again sometimes causing my foot to go off the pedal due to the contention. What could be the problem and how do I fix it?

Thanks