Intermittent Failing of Main Logic Board

iMac won't start up to desktop screen and only loads up to white screen with apple in the beginning then reboots.

I sent it in for diagnosis at an Authorised Apple Repairer and they advised the following on computer:

- Blind CSD failed CPU core current on logic board. Suspect intermittently failing main logic board.

They would not repair it as they said that Apple no longer supplies parts for this older model.

What can I do to fix this problem and is it still possible to get parts?

I look forward to hearing from you.