iMac mid 2010 monitor problems

Hi all,

My iMac monitor turns on completely black and stays black for quite a long time. I can still change the volume and play my music through the keyboard, but there is nothing on the screen. This makes me rely on a external monitor.

When I reset the pram and nvram, it temporally fixes it, but still I get this problem. Eventually my computer starts to blink and then completely turns black.

Thanks