I had a similar problem with a phillips 28 inch tv about 18 months ago. Annoyed me so much I sold it to a friend at a big loss.
I then bought a sony because it had picture rotation on the menu's so I could alter the image.
With regard to your problem, I think their may be an engineers menu(found out since i sold mine

) and if you can get the code you might be able to rotate the image. Why manufacturers hide this facility is beyond me, but fortunately sony have it on their tv's.
If you cannot find a way to get to the engineers menu, the reason the tv slopes is because of the earths gravitational pull(honestly).
If you can put your TV in a different position the screen tilt will probably alter. As a test move it and like a clock from 2 oclock to 3 oclock then onto 4 oclock until it looks right. When you get it in the right position is their anywhere in the room you can put it at that angle?
I know this is a pain, but at least it sorts the problem, because I know from personal experience it gets very irritating. I had surround sound and an excellent picture. I bought the sony, the picture isn't as good and I have the virtual surround(2 speakers) turned off as it is crap.
I hope this helps, I wish i had known the above before i sold my phillips.