How about a link to one of the sites, so we can test it, too, check out the code, etc.? It sounds like a browser problem, and testing it with numerous browsers should be the first step.

I have seen this phenomena occasionally, on other sites. It would be great to get to the bottom of it.

http://www.eyesitewebdesign.com/dale.gif

Why, I’m afraid I can’t explain myself, sir, because I’m not myself, you know...
- Lewis Carroll