The zoom and rotate functions, (for viewing) work, but at times stop working.  Then, they start working again.
I thought it might be due to rendering, settings so
-  I have tried minimizing overhead, by selecting the minimal preset.  This did not eliminate the problem.
 - The problem occurs even when my frame rates are in the range of 30
My system is a 21" desktop iMac with 8G memory 2.9Ghz 4 core processor