A wide variety of things can be responsible for such an effect, most of them are not in the gearbox. It depends very much on the conditions in which you feel this slipping, which can be the reason. If it happens often and repeatable in small gears and on inclines, then it is very likely that the chain or belt is skipping. Check the correct tension and/or wear of the secondary drive.

A further moment where this can occur is when you start pedaling again. If you pedal free after restarting to padel after coasting, it is very likely that the rear hub does not engage spontaneously enough. Please check if the chain or belt is not moving when you start pedalling again. If the chain or belt moves, check the function of the freewheel mechanism of your rear hub.

Rarely or sporadically it can also happen that a pawl is not sitting exactly in the gear teeth 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 gear. This cannot be completely ruled out in shift linkage based systems like the transmission. The transmission is designed for the resulting loads.

Please also note the following checklist.