Graphical artifacts - iMac 27 late 2009
Hi all!
I've recently been having some graphical issues with my iMac.
These are my specs:
OS X El Capitan (10.11 15A284)
iMac (27 tum, sent 2009)
2,8 GHz Intel Core i7
8 GB 1067 MHz DDR3
ATI Radeon HD 4850 512 MB
The artifacts seems to be appearing when i'm doing something like looking at photos, watching movies etc. The nature of the artifacts present themselves as follows:
1. Black windows
2. Black scrambled menus (mostly unreadable).
3. Filenames under documents are gone.
After this has happened, I usually can't boot normally anymore. When I boot the iMac, everything looks normal at the beginning, The apple logo appears as does the progress bar. But when the progress bar disappears, the screen goes grey and it just sits there, doing nothing. When this happens, five vertical pink lines are visible on the screen. Afterwards I can only boot via safe mode, and after doing so, everything looks just fine, Icons, menus, windows everything looks as expected.
The first time I had this issue was under Yosemite, about two months ago. I managed to recover from it then, but I'm not sure exactly how, since I tried a whole bunch of different things that i came up with, i.e. reinstalling yosemite, repairing permissions etc. The pink vertical lines were visible then as well.
Right now while I'm writing about this I'm in safe mode, and my desktop together with its icons and menus looks just fine. Screen updates are slower obviously.
As far i can remember, the first time it happened was when i was playing UT99 under Wine, the next time was when i was looking at photos in Photos, and yesterday it happened when i was playing a movie in vlc, only this time it happened under El Capitan.
I've googled my problem of course, and i did find a lot of similar issues, but i'm not sure what to do.
I guess my question boils down to this:
Is it a hardware issue or not, and in that case what hardware is likely causing my problems (the logic board or the graphic board)?
Thanks a lot for your help!!
iMac (27-inch Late 2009), OS X El Capitan (10.11)