Did the thumbnailer have enough time to go through all images? After reboot, did you let it idle for a while to go through?
Do all your images have thumbnails? Maybe a corrupted/unsupported image is doing it.
Did you change the camera's setting to shoot from internal to card? That could have forced sorting in an odd order.
I don't have the problem you list, however, if I open only an image or have a filter on, it only slides through those. Maybe you are using a non-standard folder? That could get them indexed but not scrolled through, say, add a "myShots" instead of using .images. I'm just guessing at this point.