There's no surefire way, to solve this enigma.
First and foremost, give us the last three numbers and letters of your serial number. You can find it in the System Profiler, or on a sticker under the battery, if it's not worn out.
There's a long way of tests. I'll divide them in software and hardware tests.
On the software side, you should redo the SMC and PRAM reset. You could also try, just if you want to feel really safe, to boot with a clean profile.
Then you must get on the hardware side.
The usual troubleshooting for your issue implies:
- Replacing the inverter cable: The telltale signs of a frayed and/or damaged inverter cable are a backlight that goes on and off if you jiggle the monitor up and down
- Replacing the inverter, who's usually the culprit for a dim or flickering light.
- Replacing the display. The display has a CCFL lamp, like a small neon, lodged in its bottom part. Usually a worn out lamp gives off some telltale signs, like a more pinkish and yellowish hue, lacking luster if compared to a new screen
The fourth cause, the rarest one, is a bad capactor on the logic board. Don't worry: that never happened in every single case I knew or read of
So, if I were you I'd try to boot out of a clean profile, maybe even from the install DVD and see what happens. If it keeps acting up, I'd change inverter and inverter cable, and only if you keep having issue, the LCD panel.