CSS Drop down menu problem

Hi, Ive pretty much completed work on a website for a client but am having problems with the drop down menu not working properly in IE (No surprise there then!). Site is built in WordPress and is based on the Thematic framework. If I revert the theme to plain Thematic, then the menu works. When I turn on my theme that's when problems happen. The menu works in Windows and Mac - Firefox, Chrome & Safari, but wont play nice in IE. IE 7 - Nav doesnt work - Wont drop down IE8 - Nav wont work on home page, but works on interior pages IE9 - Doesnt work on home page, and only drops down by one level on interior pages. Basically I need the menu to work in IE 7, 8 & 9. I am aware that some of the nicer css3 properties wont work in earlier versions of IE but the drop down aspect needs to work in a basic form at least.