Sorry, but it still jumps even with your suggestion – I even increased it to 900px but still jumps.
According to me on lots of other themes I have used, the height of the content should not have anything to do with the sticky menu. It should still work. And on this page for example (http://www.multiseal.co.za/composition), there is a lot of space to still scroll, but it jumps. See attachment
I even increased your suggestion to 900px, and it still jumps StickeyMenu.jpg