Grey screen after Antivirus scan

Hi everyone,

I own an old iMac 2008 running El Capitan.

In 2015 I had some green lines and zig-zags on screen that turned out to be caused by GPU failure. After the machine became useless with just a grey screen showing at startup, I reballed the chip and everything worked well.

One week ago I felt the machine to be very slow, it even froze during daily use. So, I thought it was the Avast antivirus to slow down the system or some malware running and heating up everything. So I performed a complete scan with Avast that found 7 menaces that I suddenly deleted. After that, the PC started to behave strangely. It froze more often, so I thought it was some "malware vengeance" and downloaded Malware Bytes to find the remaining traces of menaces. The result now is that the machine boots just in Safe Mode and the data are all there, while in normal mode it stops at the infamous grey screen. I managed to delete Avast and Malware Bytes but the issue remained.

So far I tried:

- resetting NVRAM

- resetting PRAM

- reinstalled OSX

- using Disk Utility to repair the disk

- deleting the AMD2400Controller.kext, to deactivate the former faulty GPU

... but the grey screen happened anyway!

Any suggestions?

Thanks in advance