Form With 3 Dropdown Lists - Whole Page Flickers

I have a simple web form which has 3 dropdown lists making date of birth dd mmm yyyy. The form itself is inside an update panel. The 3 lists are populated once on page load, list 1 has 31 days, list 2, 12 months and list 3, 82 years. The strange behaviour affects list 1 - (days) and list 3 (years). When I select an item from the list which is outside the viewing window via the scrollbar (20 is outside 19 is the last in the list) and press the validate button the whole page flickers. The same behavious occurs in list 3. It doesn't occur in list 2 because the viewing windown is large enough for all 12 months. If I then select an item within the window, the flickering stops.