My bike has 12 gears on the rear wheel. When I put the last gear (12) it starts constantly changing from 12 to 11 gear without me doing anything. For example, I put the last gear and it works; right away it changes to 11 gear for a very short time; then back to the last gear and so on. All the other gears work fine. I don't known what's possibly wrong. Sorry for my English.