Caching appears to be the problem. Could be on the browser level (I've seen Opera, for example, to cache too much) or something in between (e.g. a proxy).
See if adding a
Text Formatted Code
<meta http-equiv="Pragma" content="no-cache">
to your header.thtml makes any difference (it's already there in the Professional theme, btw, but commented out).
bye, Dirk