Macbook gets stuck on Grey Screen, won't boot normally I've tried many steps so I'll list them here, I can do a reinstall but I prefer to figure out the actual cause of the problem and I don't think a reinstall will solve it.

Macbook 2007, OSX version 10.7.5

I was watching a YouTube video when something weird happened to the screen and the computer froze, so I forced a shutdown by holding down the power button.  Ever since, I'm not able to boot up.

I get past the Apple logo, but it instantly goes grey.  I've even tried booting from an external drive and the same thing happens.

I've tried replacing the SSD with another working SSD, but same thing. So it can't be the drive.

I can boot into Safe mode, I've repaired permissions and disk, a restart normally still hung on the grey screen.  I can still get into Safe mode no problem and I don't think a reinstall will solve the issue because of this.

I've booted into Single user mode, ran fsck -fy and everything was ok.  Still can't boot.

Can't boot into recovery disk, same thing happens with grey screen.

Tried Command+V to verbose boot but it didn't hang on anything, it just boots and then goes to the grey screen.

I reset NVRAM and SMC, didn't accomplish anything.

Any one have any ideas?  Any more steps I can take that I haven't thought of?
I too had this problem twice. The first time the system got stuck, and switched itself off. Then later when I was trying boot it got stuck at the grey screen. When I contacted Apple for support, they told me that my harddrive is not working anymore and replaced it. All data lost. The second time when it happened was when I tried to restart the system and it got stuck at the grey screen. I tried removing all the devices connected to the system and switched it back on. This time it worked. Let me know if I can be of anymore help :-)