Re: Problems With NVIDIA GeForce 8400M GS GFX Card
I have had the same problem on my dv9000 (nVidia 8600M GS which is pretty much the same chip). It could be one of two things (I've had both happen on my dv9000, these laptops are horribly designed and very likely to fail). First it could be the graphics chip failing (mainly caused by overheating, the solder that connects the chip to the motherboard cracks under the heat until the electrical connection is broken, then your PC doesn't work right). When mine did this I had to get the motherboard replaced ($500!) and the replacement board is just as likely to fail as the original.
The second potential problem (and what I'd assume in your case since it flickers off and on and nothing crashes on the PC) is the LCD cable is failing. This happened to me as well, the LCD cable (which is a bundle of tiny wires) got stuck in a sharp edge on the hinge and opening and closing the screen caused the screen to flicker and go out (the backlight did not turn off, the screen was still lit up just no picture). I figure this was due to sloppy repair when they slapped the PC back together at the HP repair center. I took my screen apart and confirmed the problem which was right at the hinge. A new cable was $80 and thus a huge ripoff so instead I pulled the bad section into the screen a bit farther and taped around it to prevent it from bending. I then made sure that the hinge wouldn't crush the cable and reassembled the screen. This worked for a while then the problem started again and has since gone away.
I also took my laptop apart and re-applied thermal paste to the heatsinks, getting rid of the cheap hard paste they used and replaced it with Arctic Silver 5 which is significantly better. This helps prevent the graphics card from failing again by cooling it down a bit better but you have to take the laptop absolutely to pieces in order to do it.
If the backlight doesn't come on, it is most likely the inverter but could be the LCD ribbon (which also contains the power lines for the backlight inverter). If it shuts down at random times I doubt it's the backlight though.