Put the chain on the outer chainring. In the front center of the chainring, pull the chain forward. If you can see one whole tooth of the chainring, get a new chain.

The misshifting I experienced for two months turned out to be caused by a worn out chain. This chain caused both my cassette and my middle chainring to wear out as well. Having it all fixed costs too much, make sure you get your chain swapped about every 5000km.