Black screen with computer working

Hello,

I've an iMac mid 2011, 27''.

I got an issue that couldn't figure it out, even testing a lot from what a read on the internet!

It has started as blinking screen, going from dark (not complete black) to normal bright... witch lasted seconds blinking fast.

Then from a few month to here, the screen completely blanks out with the computer working fine... if connect to a TV, i can it working and test every thing...

Now every time i turn the computer on, i need to do a PRAM reset...

At start up, the gray screen most times blinks from dark gray to light gray, with no prompt sound and before the computer finish loading the screen goes black again!

Thx in anticipation!