Question iMac won't boot in anything other than safe boot ?

[Muhamad-j1234]

Hi all,
I have a 2008 iMac which has Yosemite on it. I've also tried El Capitan, but they both have a black screen when I boot. When I press shift upon boot it boots into safe boot and I can use it. Any idea why this happens? Also, in safe boot there are some white lines during animations which slowly go away and the dock looks faded. Thanks

 

[Muhamad-j1234]

It turned out to be a GPU fault. To solve it, I cooked the GPU in the oven at 200 for 8 minutes, now it works fine.