Originally Posted by cny-bikeman

One cannot diagnose and correct a problem with "a lot of the time." One needs to take the symptoms, combined with knowledge of how the entire shift system works together, and then decide what the most likely cause is. If the OP shifts to a gear and it doesn't move because the adjustment is too tight, then the same thing would occur on the next gear.

The fact that it jumps two gears the next time most likely means that the cable is binding, and the derailleur is pulling on the cable hard enough after two shifts so that it finally gives way. Imagine pulling something along a floor with a rope and it hits a rough patch of floor. You pull harder and harder, and when it finally gives way you fall on your butt, because once you overcame the initial friction the object's momentum allowed it to jump forward.