Checkered pattern on 27" iMac screen

[ascender]

Bit hard to describe, but on my iMac screen, there's a band in the middle of which needs cleaning/wiping every day. Its like someone's made a smudge across the centre of the screen, but the strange thing is that there's a checkered pattern of small rectangles through the majority of the smudge/fog.

Temperature in the office isn't a problem and I've never had this problem with any of my other iMacs, so what on earth is causing this? Very annoying while watching movies or doing work on anything dark coloured.

Any ideas?

Thanks,

Mike.