i have philips 42" lcd tv the picture goes light and dark is this normal havnt noticed this happening on my 40"tv

I have also got a 42" Philips LCD tv. 42PFL5522D

its the second one I have had after the first one blew a circuit board 3 months after I bought it, Mine aloe goes light to dark and its really starting to annoy me now, its not a gradual light to dark either, its very abrupt and very noticeable.

All of a sudden the incoming TV signal will be a bright picture and the tv darkens it down, and the opposite with darl pictures occurs.

I havetried to switch on and off all the picture settings via the menu and still the problem exsists, although this did not happen with my first one.

Any Ideas anyone?

I have also messed with every setting on the tv, i have just started to notice how dark the dark scenes really are they are terrible, cant seem to find anything in settings to adjust the backlight either