My bicycle chain slips in high gear (7th) in the rear?

I have a almost new bike that I have started riding often. For a little bit it did okay but now if I am in the 7th gear the chain slips and pops all the time. I have re-adjusted the rear derailleur and the cable also adjusting all the extremities and with the 1st and 7th gear and made sure the bike shifter properly with the chain lined up. Like I said the bike is almost brand new and done everything I know to do and read all the directions. The derailleur can wobble a little bit but i didn't think it could cause this. Any help would be greatly appreciated. thank you for your time.