Chosen Solution

Hey guys what’s up! I just bought yesterday a 2009 iMac 2,93 GHz 4GB of RAM. Previous owner said that there was probably an issue with graphic card due to small lines on the screen. So I opened it up, added a new HDD because it was removed and cleaned all the dust that was making the System running like an aircraft with fan at 100% Now it’s silent as my phone. After all that, i just reassembled everything and did a fresh install of Mac OS-X El Capitan. But after that, the computer is now crashing and freezing when I move the mouse. CPU is at 20 % something like that but it makes weird lines and sometimes fan are running super fast without reason. Here are some photos. Thanks for help

Sadly, your GPU board needs attention and it may need to be replaced to get this system running. Review this guide iMac Intel 24" EMC 2267 Graphics Card Replacement Jump down to Step 10 as you appear to have already taken the logic board fully out. With the GPU board fully removed you should be able to remove the heat sink so you can clean off the old thermal paste and apply fresh paste. See if that helps other wise you’ll need to get a new GPU board for your system AMD M76XT MXM III GPU - 661-4990NVIDIA G92GT MXM III GPU - 661-4991

not a MAC expert, but fans are normally temp controlled and the problem appears like a graphic card issue, so I would look for a device that is getting too hot to handle.