iMac 2008 screen brightness / fade problem

Hi guys,

I'm having a problem with my early 2008 20" iMac.

It's equipped with an ATI Radeon 2600 HD Pro video chip.

English is not my first language so I'm going to try to explain the issue the best I possibly can, in my own words. Don't hestitate to ask if anything is not clear enough.

The problem is that since some time (app. 1 year) there is appearing some kind of 'fade' in the brightness of my screen. It appears horizontally. The first 10 centimeters of the screen are darker than the rest of the screen, and it goes from Very dark to less dark, and eventually to the normal brightness, about 10-15 centimeters (4 to 6 inches) in.

First I was convinced that the screen was faulty or needed a repair, but since I don't use the iMac that often and it's kind of an older machine, I was not going to pay hundreds of euros to pay for this.

But last week I noticed something: when I put the imac into standby (I don't know how it is called on an English device, maybe hibernation? Standby? Sleep mode?) , and if I then click or touch a random key, it wakes up. When waking up again, the screen is fully bright for half a second, and than goes back to the faded overall brightness. This is the only possible way to ge the screen fully bright again, being it only for such short period of time.
But to me, this prooves or might be a sign that it might not have to be a hardware error, but maybe this is just some setting or driver problem. Why else would the screen light up fully again , each time it goes from sleep mode to normal desktop mode?

Knowing this, I wiped the whole drive and installed a fresh install of Yosemite, thinking maybe it got this way during the El Capitan update and hoping the fresh install would delete any faulty settings or pref files et cetera.

But unfortunately, this did not help and the subtle brightness fade / band is still there.

Does anybody have any clue what this is caused by? Or what I could do? Preferably without spending lots of money.

I have included a picture taken with my phone, it isn't perfectly clear but you can see the issue and have an idea what I'm talking about.

Thank you so much in advance for any help.
Kind regards from Belgium,

Philippe