ok....

You might have a bad power inverter board. This is the board that powers back light to the lcd.

Or....

This is what it sounds like.. Check your RAM. You might need to reseat it or it could be bad. If you have two sticks of ram in your machine you might want to try booting your machine with just one of the sticks and then swap it with the other if the problem persists. Remember that you can only touch the RAM when the machine is off... Otherwise you could damage you machine by shorting it.

Hope this helps!

Raphael