The bulk of it was caused by the fact we had an element which was only visible on-hover (a drop-down sign in menu) and within that element was an unclosed <labe

The bulk of it was caused by the fact we had an element which was only visible on-hover (a drop-down sign in menu) and within that element was an unclosed <label> tag. Because the label was unclosed, the rest of the page was being trapped in the <label> element which meant the lower half of the page only displayed when the parent element was visible (thus causing the flickering effect as the element was shown/hidden).