Bike is random upon shifting, might shift to different gear or might not. One time, for example, it might shift to the low gear for hills, another time it might stay in the same gear. I even had a 2 mile section of a ride when it was stuck in low gear - having the gear lever in all positions. Occasionally, the bike will throw the chain off the rear cogs upon shifting. Other times the chain/cog will emit a "crackle" from the rear.
My guess is the rear derailleur not working quite right, but given my experience, I thought I'd ask.
Bike Nashbar AL-1 ,Raligh M50 , Schwinn Traveler , and others
have you check your cable ? it might be getting rusted inside the housing . then if so it need replacing . the dérailleur might be out of adjusting or dirtily and need cleaning and relube . the chain need cleanig or replace.
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.