MacBook display constantly cuts out

So I know my little baby's just growing up - for the love of all that is good, my computer would be entering kindergarten if it were a human - but it's been a rough year, and I can't afford to replace her just yet.

Here's the deal: the display's started cutting off unexpectedly. It started a few months ago with random restarts, which I thought were caused by a bad hard drive, but it turns out permissions errors, stacked up over the years, really can bork your computer. I replaced the hard drive (didn't mind opening her up, since she's well out of warranty), but the problem kept occuring - only with the screen going dark randomly. I thought the new hard drive was dying, too.

But here's how it's been going down lately. About twice a day, the screen will go black. Not just the backlight; it's as though the display has been turned off. I can't wake it from the display sleep, but it's not for lack of hitting the space bar repeatedly. Keyboard backlight stays on for a second, and then goes dark, at which point, I'll force a shutdown and try to restart.

All of which is messing with my data; I've been doing a permissions repair pretty much daily now, and there's loads of 'em to repair.

I noticed today, when it happened earlier, that my hard drive is still spinning, and USB peripherals retain their power.

So what is causing my display to cut out like this, and how do I keep it from happening again (at least this frequently) until I can afford a new MacBook?

Please tell me it's not the logic board. Please tell me it's not the logic board. Please tell me it's not the logic board. Please tell me it's not the logic board. Please tell me it's not the logic board. Please tell me it's not the logic board. Please tell me it's not the logic board. Please tell me it's not the logic board.