FF vs. IE DHTML, any gurus out there?

Apologies in advance since this is a bit OT and not necessarily the right forum or board, but I thought I'd ask because of the range of expertise here, and frankly I'm at a loss...

I've run into a quirk with some DHTML coding I've been doing on an internal intranet app we use at work. I originally wrote it a couple of years ago, sort of a learn-as-you-go kind of thing and I've recently started updating it.

Without getting into long details, I had some ASP code that would generate an HTML table. The format of each table line was dependent on the data reported (ie. negative results were red text, certain lines may be with a red background) and I also coded it so that certain cells would highlight with a mouseover and bring up a relevant popup window when clicked. The original coding was a bit rough, and I relied on font color and bgcolor attribute tags for the original rendering, and then "this.style.xx=yyy" coding to handle to mouseover / mouseout events. Worked fine.

As part of my update (I'm porting it to PHP), I decided to clean up the html and started using CSS. I created classes to replace the attribute tags, and the mouseover/mouseout events simply altered the class of the current element (and sometimes an adjacent one). It still works as expected, except that the performance in IE is very sluggish now on tables greater than 20 or so rows, and some of the reports generate hundreds of rows (by sluggish, I mean a little slow to scroll, and the highlight effects happen a second or so after the mouse is over the appropriate cell). There's no other client-side code on the page running that should be dragging down performance.

I tried running the same page in FireFox, and it worked exactly as the previous version did, no sluggish performance at all. Accessing the page from IE on my work desktop (XP-based vs. Win2K) showed the same symptoms, the page was sluggish, so I'm ruling out a problem with my local system settings.

I also checked taskmanager to see if there were any problems with system resources when IE was accessing the page, but my system was showing idle, no cycles being used by IE.

So, basically, I'm at a loss as to how a page rendered with html tags vs. class settings can render identically, but impact the performance of IE that drastically, particularly since it works fine in Firefox. FWIW, the new version of the page actually renders faster (thank you PHP), and for all intents and purposes works, it just drags the browser down.

Again, sorry for the long-winded post but I'm at a loss. Any advice would be appreciated, or even if anyone knows of any appropriate forums or resources I could try on the web...

Thanks,
KV