[PMSHIVAPRASAD]

Recently, i am observing that my BB9900 display goes off and come back after few minute. I had gone blackberry service in India (Bangalore) and got my software reloaded but issue repeated after 3 days. Hope this some related to product issue since one of google search mentioned same.

[tsac]

There is a light proximity sensor that may not be working correct. Was the phone dropped or was it ever wet? Depending on the amount of light or if the sensor sees light or not it may cycle but generally not continuously.
You can try a reload of the OS but if that does not workthe phone may be defective.
Last resort is to delete any added third party apps to see if one is causing this.

[rambo47]

Some screen protectors can cause this. If your BlackBerry has a screen protector installed and it's either mis-aligned or is not made of the proper material it can make the BlackBerry think it's being held up to your face as if you're on a phone call.