Dell Studio 1747 :: Not Booting / No Display - With External Screen Works Fine

my studio 1747 (not in warranty) (i7 win7),has stopped booting anymore it was working fine till last week ( occasionally my screen does flick in last coupon of month), So when I do turn my system on all upper volumn light flashes from left to right and then right to left and then system turn off itself, during that period screen show no display.

connecting external monitor (TV via hdmi) does work and system boots fine show display on external monitor but no display on system LCD. Something display flicks on external montor also, it happens when my laptop LCD tries to display something (i.e. sometime laptop LCD flicks and sometime it display for few seconds and then go back to Black screen). Also when connect external monitor my mouse pad stops work and sometime works when I connect usb mouse, but when LCD screen start display something, mouse pad also starts work.I tried all stuff, whatever I could find on internet as below  

1) Taking battery off and press power button for 1 min.  
2)  Hold 'D' key and then press power button LCD sometime show some color and then goes back to black.  
3) Removed battery and CMOS battery to total drain of charges.  
4) Removed keyboard panel and checked LCD display cable and make sure its not loose.

To me it seems like motherboard, graphic card are fine as system is working on external monitor, LCD screen also looks fine as sometime is display and not greying out (its either Black-out or display). As I have also checked cable is properly attached with motherboard, so it can't be loose connection. Don't understand why LCD works sometime.