Behaviour in almost every case is not a faulty electronic component but rather a contact problem, either on the PCB or on some connector including and in particular the IC sockets.
I understand that when you start up the Matrix it runs fine as long as it is cold. Why not try to induce the stuck problem by tapping on the PCB, connectors and sockets around CPU. This would indicative of a contact problem.

I assume, you do not have access to a decent oscilloscope to check whether or not the CPU is still running when stuck occurs.




Good luck