Scotty's V3 gallery - jumping dragger

I’ve modified Scotty’s gallery (V3 with thumbs scrolled) to hold larger thumbs (as well as other changes) and I’m having problems with the up and down buttons making the dragger scroll only halfway down and then stop. If I click and hold on the dragger to move it down starting from the top, it will jump halfway down. I’ve looked at the scrollbar AS and I can’t find out what’s wrong. But I’ve modified the file so much, I might have missed something.