Have you done a proper gear adjustment after replacing the ring, if not then you should start with that. Make sure the limits are set correctly (both front and rear gears). Also check if you're using the correct chain size for your setup.
The problem could also be a change in the chain tension, as the rear derailleur is now working with a larger cog. Check if derailleur is properly adjusted and maintains the correct tension. Watch for any sudden chain 'jumps' when shifting gears, if that occurs then get a £7 chain tensioner from ebay (the one with jockey wheel and tension spring, not a fixed one as shown on this video
) and install it near the front ring, that should solve the problem.