Honestly I think kur4o is on the right track. Sounds like it's an age-related failure of the socketed part. In general, these PCMs are insanely robust and don't just 'fail'. They can be bricked by a bad flash, but now that Flashhack exists the chance of that approaches zero. So nowadays, a stock unsocketed PCM is essentially a non-issue.

So that being the case, I too would look towards the custom work surrounding the socket and see if something has broken. Maybe reflow the socket, see if it's just cold solder joints.