cfchart image expired (but only in IE!)

I'm running Coldfusion 8, in a clustered deployment on our live server (Windows Server 2000), and standard edition on my local machine (Windows 7). We are using IIS on both machines. This issue happens in both places.

The front page of our application contains a cfchart image. In Chrome and Firefox, it displays just fine. In IE (8 and 9), it seems to be a random chance whether the chart will display or whether we'll get the dreaded "Image Expired" message instead. Refreshing the page DOES NOT cause the image to return. Sometimes, navigating elsewhere on the website and doing some form submissions, then coming back to the home page will make the image return, but not always. And sometimes just sitting on the home page and refreshing will make the image expire.

We have edited webcharts3d.xml and bumped the minTimeout to 180000 and the maxTimeout to 300000. (This is what appears to have fixed the issue in FF and Chrome.)

I had read that clustered deployment could mess with cfchart so I thought the problem might be related to that; but when I installed standard edition locally and tried it, I got "Image Expired" there as well.

So, does anyone have other ideas? My boss is breathing down my neck and I don't have an answer to give him.