The first thing to do is to look for a stiff link. This is not so easy to find, but if you are meticulous, you can find it. You don't have to take the chain off of the bike. When you find it, flex the chain back and forth sideways - the un-natural direction. That usually works. If you do find a stiff link or two, this might be a good time to clean your chain.
Otherwise, check your shifters, then your cable as suggested by bikeman 715.
Good luck. It seems so frustrating now, but your problem has a pretty easy fix.