Hi John,
Apart from the hints from Ron in an earlier message in this thread, I have not much of any value that you can try, except for making sure the temporary and cache files are cleared from those users browsers. You could even make a copy of your project and deploy in another virtual folder and see if the same users that currently have problem on your current version, still have it on the new virtual folder.
It might also be an option for you to update to 6.3.8a, or wait a few days for 6.3.9, and see if it still occurs.
These are all just ideas for something you could try to troubleshoot, but it might help you and us find what's really causing it. Try isolating browser versions and user security levels and try to spot any patterns that might help.
Please let us know of any progress, and if you think that we might be able to reproduce in our environment, then we'd be most willing to try any minimal sample application for you, but as you describe it, that app would probably be one single combobox, so it wouldn't help much !
Palli
Palli