I'll try hack something together by tomorrow. I had hoped that someone already knew this problem to be honest

I doubt that the the problem is that the view doesn't refresh itself - other progress bars carry on the very next instant. Since I'm downloading multipart files from a newsgroup over a 20 Mb connection, the progress bars go very fast since the multiparts are generally small. It's a freeze for the moment that the rows are appended to the model. I suspect that the model is turning off updating internally while the rows are being appended and it's in that time that the multiparts complete. Since they are finished by the time the model enables updating again, the download threads have already moved onto another row. But I shall try and provide something compileable, although it could be difficult. If you want, you can grab the code from svn trunk from sourceforge : https://knewz.svn.sourceforge.net/svnroot/knewz/trunk
I realize that it is a whole program and therefor not ideal to track this one specific problem, but I think it would be hard for me to code something artificial that could reproduce this problem.

/edit : you will need cmake and the kde4 libraries to compile this.