Chain Slipping

[durham2]

Hello to all bicycle enthusiasts. I need some help on my mountain bike that I have recently purchased. It is a Cannondale Trail SL 6 and I am having some problems with my whole drive assembly. The problems that I am having are when going up a hill, not shifting down into a lower gear, my chain feels like it just pops off the rear cogs and I inturn feel like I am about to kick my pedal off the crank. I notice it when I have it in on the third lowest gear on the rear cassette and the second gear on the crank. I have looked at the alignment on the rear deraileuer and it looks fine to me. I am at a point of don't know what to do. If anybody could give me a hand that would be great. Thank you for your time.

[cyclerUK]

Chances are the chain and sprockets are worn.
Measure the chain as per:-
http://bikeride.com/chain-wear/

If you do need a new chain then probably the sprockets are worn as well. If the chain is badly worn the the chainrings could also need replacing?

[trevgbb]

If this is a new bike, return it to the supplier for sorting out under warranty, if you bought on line, get it fixed at a LBS and send your supplier the bill.