My chain on my 9 speed is slipping My chain on 9 speed is slipping. I measured my chain but the slipping continues.
By the look of things, you may have the cable a very little bit too tight.

In other words, after you have gear down, the cable is still pulling just enough to want to go further (in the direction of the spokes), which causes the chain to ride up on the gear teeth. 

Adjusting the cable adjuster (sometimes it's located right where the cable meats the derailleur) a 1/4 turn to loosen (make longer) the cable, should be enough to drop it back over the teeth.

This off coarse is assuming that by your description the cassette and chain are not worn or have not be independently replaced.