As you stated the issue stops once you restart the QlikView services, likely running into a system resources issue.
Please note that 11.20 SR5 was released in December 2013, so it is 3 years old at this point. Qlik has made many improvements in the functionality of the QlikView components, so would suggest you upgrade to the most recent version - v11.20 SR15 or v12.10.00, to take advantage of these enhancements.
The first step is to configure the QlikView Server Events log to "High Verbosity" and split "Daily", if not already configured that way. This will allow you to review these log files easily. Review these logs and post the most frequently occurring errors here in this post.
I would also encourage you to contact Qlik Support for assistance.