There's an issue in the beta as well as the current live release in which if you try to cycle images in the editor too fast the application fails to render the next image, the overlay that says "loading..." just stays like that and fails to move on so you have to restart LR. I notice it more and more on 40D images (raw) but only a minor few times with 350d images. A friend also reports the same issue and he has a 5D whilst someone he knows has a Nikon D3 on MAC and has no problems so obviously Adobe need to work on something here.