Chain is choking and pedals fly forwards when going up hill or standing on the bike pedals I have a 2 month old Vodoo Bizango from Halfords in the UK. I have had this problem about 2 weeks into having the bike where if I'm on the highest gear and want to do sprints (standing on the pedals) the chain will choke/make a churning noise and then the pedals will just fly forwards, I have not been able to replicate the same thing when I turn the bike upside and lay its on it's seat. This caused one of my bike chains to snap, supposedly they replaced the derailer and the chain but I'm still having the same problem. It's now started to creep to lower gears now when I'm climbing up hills too. 

I'm a complete novice at bikes and the mechanics but I need this sorting, it's my main mode of transports especially to work. 

All help is appreciated, thank you.
Sounds like the problem might be that your freewheel isn't freewheeling.