Highest gear "jumps" under load
It wears fast because there are fewer teeth in contact with the chain, but that might not be the cause. Here are some other possibilities: 


A stiff chain link is most noticeable with a small sprocket. It's also more annoying if not more noticeable when under load. This tends to cause an even rhythm under steady pedalling, about one every 3 pedal strokes depending on your chain length and chain ring. 
The high limit screw needs a tweak for some reason and the chain is trying to change gear. Even the chain tension is a possibility. 
The derailleur has taken a slight knock and isn't quite straight. 


The first two of these are best checked on a stand, turning the pedals by hand and watching the rear mech.