iMac 2010 comes up with blank gray screen

Hi

thank you for taking time to address my problem

I have an iMac 2010 that sometime gives me a blank gray screen after the OS bar is filled up. I've had this issue twice before, the first time it went away by re-installing the OS, after it happened the second time, i though maybe the HDD is going back, so i replaced the drive. Unfortunately a few weeks after changing the HDD and re-installing the El Capitan and restoring files and apps via original HDD the issue re-appeared.

Im back to square one with a blank gray screen. I decided to post here and see if you guys can help me out.

The can boot in the Safe mode (SafeBoot) and there are some Diagnostic reports but i dont have enough knowledge to interpret them. Hope the community can help me figure out whether the issue is related to a program or there's a hardware issue with ATI radeon chip.

Thank you for your help