you could move thw wheel as far forward as you can and take a single link out of the chain, but i had the same problem with my bike before, it turned out that the freewheel was slipping, so i replaced it with an ACS Freewheel, that was about 5 months ago and it's been fine since. if the problem happens again possibly try getting your mits on an ENO freewheel, they're the most reliable freewheels on the market, but will set you back about 80 big ones The reason it slipps is because you've got a small chainring you get a lot of torque which puts a lot of pressure on the freewheel and can cause the teeth to slightly round off and they won't lock into position properly when you pedal suddenly, causing your cranks to spin effortlessly and sending you knee into your stem at hight speeds resulting in a hell of a lot of pain and a day or two of funn walking. talking from past experiences Good luck getting the problem sorted anyway mate! Mike!!!