Strange File Download

Users of my web site, running MD-Pro, a PostNuke (php-based) clone, suddenly started to report difficulties downloading files from the download module. We assumed it was the download module and spent ages diagnosing it, uploading fresh versions etc. After some time someone had the bright idea to see if the files could be downloaded direct (using the URLs) to eliminate the web application.

We found: Some users can download all files from the server with no problems.
Some users can download some files but not others.

So far as we can tell only MS Word files are affected, but not all.

The files on the server are not corrupted and there are no permissions issues

Disabling antivirus and firewalls locally makes no difference.

REloading fresh originals to the servedr does not help.

The host reports that no changes to the server have been made recently.

We have discovered that files in the web root are OK. The furhte down the directory structure they are, the less likely they are to download. Performance varies according to the browser in use. According to the host technical support, 'Firefox appears to be returning the data from its own cache. IE is only doing so for root. For the other places IE tries to download the file but stops receiving at exactly the same number of bytes from different locations.'

Using another server on a different host, the problem disappears so it must be due to the host setup in some way. Diagnosis is difficult because the host technical support can't reproduce it.

Platforms include Mac, Windows and Linux and browsers include IE7, Firefox and Safari.

One user with Mac isn't having problems, another with Mac is. All others reporting problems are on Windows. Users are at different locations, using different ISPs so it is unlikely there are common local problems.

The only common element I can see is that the ony files causing trouble are MS Word, though some of these do download OK

We have run out of ideas why this should be happening. How can some users have problems with the same files on the same server and others not?

What could have happened to cause this problem on a site that has been functioning correctly for several months on this server?