This is usually related to software conflicts, unusable files or incompatibilities on your personal computer system. It can also be related to specific hardware combinations, and sometimes the way in which your hardware is configured. There may not be just one particular cause or solution. Please check out our QuickTime help section, it is possible that you will find the answer you need there. Since there can be many different causes for a particular symptom, some suggestions may not work, and you may and you may need to discover a solution on your own.
If none of this helps, please go to Apple's QuickTime discussion Forum (here). Leave a detailed message in the appropriate section and you're sure to get some helpful response.