We are experiencing something very similar. On average every 24 hours the server fails to render the selected qlikview document, instead displaying a blank white page.
The only way to fix the problem is to go and restart the QVS service or kill the QVS.EXE specifically.
From my investigations it appears the bug is triggered by a user opening the document, as the web server log always shows someone trying to load up the document and then getting timeouts etc.
In our case QVS.EXE is holding open a file lock on the qlikview document which is preventing any further access to the file from users or reloads. By killing QVS.EXE it releases the lock and allows the system to get moving again.
The other thing I noticed is QVS.EXE normally rounds around 1.2gb of RAM all day long, however anytime the freeze / filelock occurs I can see the memory has jumped anywhere from 2gb+ up to 5gb.
Given the symptoms above I can only conclude some sort or memory leak or race condition is occuring in QVS.EXE.
I am in the process of raising a support case with Qlikview now.
Hope this helps.
We ended up restructuring our folder structure within Qlikview, we had several mounted folders that had caused the same document to be visible twice.
After cleaning up the structure to ensure each folder/document was only accessible via one unique mount it appears to have fixed the problem.
Hope that helps.