A wide variety of things can be responsible for such an effect, most of them are not in the gearbox. What exactly causes this effect, depends very much on the circumstances under which this skipping/slipping occurs. 

If it happens often and reproduceably in small gears and on inclines, it is very likely that the chain or belt is skipping. Check the correct tension of chain or belt and the wear of the secondary drivetrain. If a belt or chain tensioner is used, incorrect positioning and/or adjustment are possible reasons for skipping.

Futhermore skippings can occur when you start pedaling again after coasting. In this case, it is very likely that the rear hub does not engage immediately. Please check if the chain or belt in this situation is moving or not. If the chain or belt moves, check the function of the freewheel mechanism of your rear hub.

Rarely or sporadically it can happen that within the gear box a pawl is not sitting exactly in the gear toothing after a shifting operation. This can remain stable for some time after the shifting process, but at some point, during a slight change of load, the pawl may slip off and slip into the next toothing. This happens typically with a clearly noticeable noise. The free path of the crank is usually merely about 15°, when this happens. This effect cannot be completely ruled out in shift linkage based systems like the transmission. However, the transmission is designed for the resulting loads.

Please also note the following checklist.