iMac display has discolored tan lines I have a 24 inch, mid-2007 iMac purchased used. I don't know the original owner and it's out of warranty. 

On the display is a series of tan or yellowish horizontal lines - all an approximate inch apart, for a total of about eight or so. The spacing between is regular. They're wider (about an inch wide) at the left and right edges of the screen and taper to nothing at the middle of the screen. They are particularly noticeable on light-colored or white backgrounds, such as a Pages document, most web sites, etc.

I'm certainly open to disassembling the iMac and getting at whatever it may be (the onboard GPU?) that's causing the problem. In fact I have opened the machine for an HD replacement and fanned and cleaned, but to no result. 

I also use smcFanControl to boost the fans, as I thought it may be poor cooling in the machine, but this has no effect.

Does any one have ideas or suggestions as to what is causing these discolored lines?
This looks like a backlight uniformity issue.

This is where the screen isn't being evenly lit by the CCFL bulbs behind it. Some of them are outputting differing amounts of light and cause this perceived 'discolouration'. The lighter colours really do show it.

Unfortunately, this is one of the disadvantages of a CCFL backlight system as opposed to something like LED backlighting - backlight uniformity can become an issue, especially with time.

You'd be looking at a screen replacement - outside of warranty, that's gonna cost you.