If you changed MPU to an earlier one, you also need to make sure the OS and the BIOS match, it might be why that didn't work. Since you mentioned you had already replaced the power supplies, I figured you had a set of known working supplies. The power supplies need the fans, if they overheat they will cause all kinds of issues. Although the fact that your mpu is freezing up, my money is on the power supply of the mpu. When you power off the mpu but leave the machine on, the machine looks the same as it does when your system fails right? You can turn the mpu off by using the little toggle switch on the MPU if you haven't tried that already. Next time it freezes up, try removing power from mpu only and leaving it off for about 20 minutes and power up, but leave the rest of the machine on. If it powers up right, then you know you have an mpu issue and the rest of the cabinet is probably ok.