I got this weird problem with my laptop. It would function normally for some time before the screen displays all sort of weird colored lines and scrambled colored dots finally ending at a black screen. None of the keys would work so removed the battery to shut down the system. All the diagonostic tests showed the sysyem is fine.
Finally tried all your troubleshooting procedures but the situation worsened, now it wont even run the basic diagonostic tests (system dies before the tests are completed). I tried and failed to install the windows 7 again with the cd that came along and from other sources as well. It doesn't work more than 2-3 minutes.
I experinced the same problem before the warranty period was over and the dell guy came with a new motherboard but the problem did'nt show up at that time and he updated the system without replacing any hardware part. Now i'm 6 months out of warranty and my laptop is as good as dead.