My bike is 7 months old and I ride it to work and back 5 days a week (16 miles a day) in the last two weeks the chain has started to slip when ever I go up a slight hill or start off in anything over 15th gear. I clean and lube the chain regularly and I noticed today the chain is not sitting on the chain ring properly. The gears change the same as the day I bought it.

Any thoughts?

Thanks