iMac freezing up

Hi,

I have a 20 inch 2.66 Ghz (Early 2008) iMac. It recently started to freeze up for no apparent reason when using it. Now it won't even boot up completely. It freezes up when reaching about 60% of the progress bar during booting. I have to force a shut down. 

I tried the following:

• deleted all login items
• disconnected all peripherals
• ran Disk Utility First Aid
• upgraded to Mojave
• booted from an external drive

None of this stopped the freezing. However, it boots up in safe mode. It might be a hardware issue but I can't figure out a way to diagnose the problem properly. It won't load Apple Hardware Test or Apple Diagnostics.

Is there anything else I can do?

Thanks