You do not show enough of the context in which this code is running, so it isn't possible to give you an answer.

I don't know how you retrieve the frames from the external source, but you could have a race condition. The fact that your freezes occur sort of randomly implies some timing issue like this.

Instead of calling findChild() every time you want to update the pixmap, why don't you simply save the QLabel's pointer when you create it?