It could easily be gunk in the chain and cogs. Clean off the gunk with brushes and solvent (messy job) and then oil the chain. When things look clean, stand your bike against a wall and run the pedals backwards. The chain should run smoothly through the cogs. If it makes little jerks as it goes around a cog or through the jockey wheels, you have a stiff link in the chain. Your LBS can fit it in 2 minutes, or get yourself a chain tool and do it yourself. Other things to check: remove the chain from the front chainrings and grasp the two crank arms. See if the crank arms move relative to each other. See if the crank arms move in and out relative to the bike. Check that the chainring bolts are firmly tightened with no play in the chainrings relative to the crank arms. Check that the rear derailleur had no sideways play but moves front-to-back on it's spring. Since your LBS recently checked the bike, these things are unlikely, but occassionally something will loosen unexpectedly.