Thanks for this.  I think it could be the ballast as well.  Others have suggested that the bulb could be failing but I think if the bulb goes, it goes, and it is not a gradual fading.  
 
	the  other thing about this is that it seems like when the ballast warms up the light goes back to white so that I think is an indication the ballast is starting to fail.  Would you agree?
 
	Dave