Gateway 400 2.9 pentium 4 screen problems

Hi I have a gateway 400 laptop with windows xp home. when you start the computer up the screen starts out almost like you are looking through a negative. There is still some color but the screen is very dark and hard to make out the icons. Then the screen will fade in to perfect and then about 15sec later it will fade back out. It will only due that once unless you put it in stand by an then take it out of stand by then it does the same thing. If you can hook up a external monitior with no problems. i changed the LCD inverter and that done nothing. the backlight seems to be ok. I assume if the external monitior works than the video card is ok. I assume if the screen fades into perfect than the screen is ok. Please help signed Jason