iMac (mid 2011) doesn't detect second display after update to 10.8.4

I use a NEC MultiSync PA271W connected to an iMac (mid 2011) via minidisplay port. I got the monitor about 5 months ago and had absolutley no problem until today and after I updated mac os x to 10.8.4. all of the sudden my NEC display cannot be detected by my mac os and I get "Display Port no signal"!! I went through all mac forums and tried pretty much every solution I came up with. including unpluging the iMac and monitor, ctrl+shift eject, etc. none of them worked!! the strange thing is from time to time the second monitor get detected by mac os and then after couple of minutes, without me doing anything, it the monitor starts flickring and when I turn it off and on again the picture is gone again!! It's so fraustaring considering I am using the original NEC Display to minidisplay cable and everything was just fine until today.

I appreciate and helps from you guys.