Originally Posted by 9tee7pony
My assumption would be that they have a connection or a wire that is falling apart. I know on mine if one of the wires that sticks out below the cluster isn't just right one of the sections won't light. You should also check the ground wire.
+1 on the connections.
Always check your connections first, if they're working some of the time, there's a good chance you have a bad connection and or ground connection.