iMac late 2009 black horizontal lines

[Rorenz]

Hi everyone,
I have a late 2009 iMac which was repaired about 2 years ago because of the GPU (it had high sierra installed). It was baked since because pink vertical lines appeared all over the screen and I could only use the mac in safe boot. On the same occasion we replaced the internal HD with an SSD (Samsung 850 EVO) and installed Sierra.
After 6 months the mac displays the same lines and About a month ago I've followed the bake guide and applied the new thermal paste and installed Mac fan control and all was ok.
Two days ago I've decided to update the mac to high sierra since I wanted to try to install the Mojave patcher. After the first boot the mac was ok the only minor defect was a tic coming from the left speaker when the audio was required (a boot sound, music playing,...).
I install the latest security update and the at the next reboot this time I get displayed black horizontal lines and cannot boot the iMac.
I've tried:
1. Restore from latest backup (10.13.6 no security update)
2. Initialize the whole mac and install high sierra (from usb and from recovery)
3. Restore from a backup with Sierra
4. Initialize the SSD and proceed with a clean install of Sierra

Same problem as before black horizontal lines as shown in the picture, so I'm thinking that now high sierra is not the problem.

I've had a bad relationship with high sierra since the 2 times my mac goes insane I've recently installed it, now my question is: am I very unlucky and the GPU is gone again the moment I update the iMac or did High sierra contributed in any way to my situation?
Should I try a third bake?

 

[burka_tamas]

Gpu gone

 

[retta283]

At this point the chance of future failure is more likely and every time it happens it will be more likely to again, and it won't take as long to appear. I'd seriously consider upgrading your machine to something newer, as your GPU will not stop failing.

 

[r6mile]

Baking the GPU is only going to be a temporary fix, and as you've seen it has failed again. You should just replace the GPU; there is a whole thread about it, the K610M will have similar performance as your current card, is the cheapest way to go if you are just looking to get your computer back up and running. Other options can be had for more money if you are looking to get a performance improvement.