Flashing Question mark after clean install

Hello All

iMac mid 2011, 27inch, 2.7ghz, i5 quad core, 4gb ram

I am looking for some help on this one as Apple just completley refuse to help and the Apple Store near wont even look at me.

I was give the above iMac by my best friend as he bought his daughter a new 2017 iMac

1. I upgraded the iMac to HighSierra.

2. The iMac was extremely sluggish, took forever to boot up etc

3. I tried to boot off my ebootable usb thumb drive which had High Sierra on it

4. I was greeted with a firmware lock icon.

5. Asked my friend what the password was and he said he didnt even know it had a firmware password

6. Never mind i was able to boot up off the usb thumb drive after selecting it startup disk in preferences

7. Bought certified apple ram and installed it, iMac recognises it no problem

8. iMac starts restarting itself randomly even while not even working on the iMac

9 Removed third party ram and went back to factory configuration

10. Thinking it might be a glitch with the os, i formatted the drive and started to reinstall the os, but the imac restarts halfway through installing the os

11. Kernel panic, imac just restarts itself in a continuous loop, says os not yet set

12. Removed hard drive, connected it to my sons mba2013, successfully formatted the drive, installed el capitan. Checked to see if it would boot up my sons MBA2013 and it does so succecfully.

13. Re-installed drive into the iMac

14. No start up chime and all i get is the blinking question mark in a folder.

15. Tried to boot in recovery mode (command-r and also command-option-r), goes straight to firmware lock screen.

I am out of ideas what to do now. I double checked that the drive can boot up my sons MBA2013 and it does so all the time everytime, just when I put it back in to the iMac i get a flashing question mark

Sorry for the long post but I am out of ideas and Apple will not help as my friend does not have the original receipt for the iMac. Thanking you all in advance