Continuous reboot even after replacing Logic Board

iMac 24” – early 2008 - Mac 8,1 – model A1225 – EMC 2211

S/N: QP82805M0KM

HDD – Hitachi Deskstar 1TB – Apple firmware 2007

Problem:

Apple logo – chime – progress about 50% then reboots….. and again….

Procedure:

• Reset SMC & PRAM – no change
• Tested memory modules individually – no change.
• Safe-mode – freezes
• Options – select Recovery disc – voice to select language (English) – then freezes
• Use USB with full version ‘Mountain Lion’ – boot freezes
• Recovery mode – reboot at 50%
• Single-user mode – boot freezes
• Option ‘D’ for hardware tests – boot freezes

Used USB with ‘Mountain Lion’ installation files

Get into Disk Utility and Terminal

Use ‘fsck’ utility – “HDD is ok” …. “Volume has been modified” – repeated until all ok.

Reboot – boot completed – graphics background appeared but no icons.

Icons appeared after about 15 minutes.

Repaired/verified permissions etc

Machine working - OK

Three days later –same problem.

All boot options tried, plus booting to Internet for recovery – all as above

Tried with USB - Full versions Mountain Lion/El Capitan – as above

Tried with USB – Lion/El Capitan installation files – as above

Single-user mode – ‘fsck’ – HDD is ok and “Volume has been modified” – repeat until error clears but it reappeared after every attempted boot.

Tried all possible Boot options, also with various usb devices.

Attempted Internet Recovery using cable-connection – no success.

Firewire connection to MacBook Pro – Target mode - reformat iMac drive.

iMac re-started - Attempt made to re-install the OS over the Internet – failure.

Re-connect Firewire – Target mode – install El Capitan – reboot iMac – reboots at 50%

 Purchase and install Logic Board – eBay UK – ‘respected’ supplier, includes test report.

Boot – then the video fails (!) – graphics for a few seconds, then black screen.

Remove GPU card – remove & replace thermal compound – reboot – Graphics ok (should have done this while changing Logic Board !)

Exactly the same as before – logo & chime then reboots itself at about 50% of the boot sequence.

Tried booting from Lion & Capitan usb’s, and Lion & Capitan installation usb’s – rebooting at 50%.

About the only thing left is the actual processor – but surely if it is a processor problem I wouldn’t get the boot logo or chime, or be able to use Target mode ?

Any advice, suggestions, or information greatly appreciated – this is driving me mental !

Note: I’m a retired electronics and IT engineer – many years repairing PCs, just over 2 years repairing Macs - have repaired 54 Macs, including 12 old iMacs …..

I hate to be beaten – but maybe this one…?

Thanks in advance - Peter