Keyboard backlight going on /off

I'm wondering if anyone can help me here. The keyboard backlight for my computer keeps going on and off no matter what the ambient light is like. Have had this problem occasionally for three or four months but today it is going on and off every few minutes. Haven't done any major updates lately or changed anything that I know of. Anybody have any suggestions? The comp. is about three years old and still treating me great beside this.

Late 2008
2.4GHz Intel Core 2 Duo
4gb Ram
running OSX 10.5.8

Thanks much.
michael