I think it may be an inverter issue if its related to the back light or possibly the back light is going out like you said, i did not see that you wrote that before I must be tired! anyway apple suggests this for troubleshooting a dim screen.

1 Adjust screen brightness setting.

2 Verify that backlight cable connection is secure.

3 Check display cable and inverter board connections to the

logic board.

4 Verify that cables are not pinched or severed.

5 Replace inverter board.

6 Replace display.

7 Replace logic board.