What the... unexpected behaviour for CSS buttons in IE!

IE! Aaaaarrrrggghh! (grabs sword off the wall ready for slaying).

Ok check out http://www.designgeezer.com/extranet/acm/ which is the Academy of Motorcycling in test mode. Basically my link, over, and active states for the buttons are contained in the same graphic so all I’ve done is change the background position. So why, oh why, does IE show a white background on rollover before displaying the right rollover graphic state? Doesn’t do it in FF! Lovely, sweet, friendly Firefox…aaah…how we love you.