This happens if updating the preview gets too slow. First it reduces the quality (which your image shows) and then it stops updating to keep the print healthy. If updating gets slow the host can not send commands fast enough any more so it would reduce print quality and we do not want this.
In Preferences->Basic Settings you can disable quality reduction but I'm not sure if this will also prevent stop rendering, but I guess. Any how I warned you what can happen if you do :-)
Repetier-Software - the home of Repetier-Host (Windows, Linux and Mac OS X) and Repetier-Firmware.
Repetier-Server - the solution to control your printer from everywhere.
Visit us on
Facebook and
Twitter!