I have developed a website that works perfectly in IE6, Safari 5, and Firefox 3, but is totally unstable in IE 7 and 8.
(See, for example, www.andynic2.eu and then click "Sculpture and Paintings".)

Each html file passes validation on the W3 validator. The heading in each file is:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">

Any idea what I need to do to get the application to work correctly in IE7 and 8?

Thanks for your help,