dim display

I have a MacBook Pro 17" and the screen is dimming with the 'footlights effect' on the bottom of my screen. Now normaly I would think this means the screen is shot BUT... When the MacBook wakes from a restart the screen is nice and bright and the footlights are not there... but this is only for a second or two then the screen dims. Its not dim to the point where it is not readable but certainly not as bright as it should be, or used to be. Yes it is an older MacBrook from late 2008.

I have brightness set as high as it can go and I have unchecked the auto adjust to ambient light in the system prefs. I have reset the PRAM and PMU.

Specs:

MacBook Pro (17-inch, Late 2008)

2.5 GHz Intel Core 2 Duo

4 GB 667 MHz DDR2

320GB HD

NVIDIA GeForce 8600M GT 512

A1261

OS 10.10.5

Any ideas? Id like to try everything before I attempt a screen replacement.

Thanks in advance,

Joe