flickering and jumpy header repaint on menu selection

my site, innovations commercialization uses a simple background picture and menu items. it is “sticky” to the top of the page. whenever the menu items are selected, the page title and background flickers and jumps one line before settling out correctly. this happens at all page widths.

Is there something I’ve done incorrectly in settings? I see no mention of it anywhere.

is there a way of keeping the header block stable with no re-write and simply updating the body?