MacBook gray screen

I have an early-2008 MacBook with 10.7.5 on it. I also have 10.6 on it for the old look. I can't boot in to 10.7 anymore, it hangs on a gray screen after the Apple logo shows up. 10.6 runs normally, and 10.7 will run if I hold the shift key on startup. I can't reinstall since 10.6 refuses to run the 10.7 installer, (log says a certificate expired, but even setting the date back won't do anything)

This gray screen has no cursor and nothing works, sound or brightness don't change, but the display dims after a while and shuts off a little while later. I have idea what is going on or how to fix it. Recovery mode is not available on my Mac's model since it's too old.

Edit: I forgot to mention that the power button turns the screen on again when it dims and shuts off, and pressing any button brightens the screen back to normal, And while in Safe Boot (holding Shift) the login screen freezes after I enter my password and hit return.