I was looking into the issue you are describing and think I have a great article to help troubleshoot it. The article is called Apple computers: Troubleshooting issues with video on internal or external displays found here: http://support.apple.com/kb/ht1573.
Start here, and work your way down as needed:
When using an external display be sure to check the following:
- If you're using an Apple notebook, confirm the AC power cable or adapter is securely connected to the computer and the cable providing power to the display is also secure. It is always good to have your notebook connected to AC power when an external display is in use.
- Confirm display adapters are fully seated in their respective connections and that they are supported models and for the computer and display. Refer to these articles to assist you with adapter compatibility and further configuration information:
- Remove all display cable extenders, KVM switches, or other like devices and retest to determine if the issue is resolved.
- If more than one video adapter is in use—or "daisy-chained"—troubleshoot by using only one adapter.
- Example: A mini DisplayPort to DVI adapter connected to a DVI to HDMI adapter is an unsupported configuration because there is a series of adapters in use.
- If available, try using a different display and or adapter (or use a different connector by using DVI instead of VGA, for instance).
All the very best,