Screen goes blue and loses display intermittently

[Blue Powell]

More frequently now, randomly, the screen display is replaced with a bland blue screen (not screensaver) and the current display is lost. It lasts for a short time then the screen display comes back. How to stop this?

 

[honestone]

What type of machine you are using, and also what kind of external monitor? (What OS you are using might help also).

 

[Blue Powell]

Thanks for your response!
The computer is an i Mac desktop (monitor is the computer therefore integral) 21.5 inch late 2009 3.06 GHz Intel core 2 Duo Memory 12 GB 1067 MHz DDR3
Software: Mac OS X Lion10.7.5 11G63b