A4000T Problem

[Zac67]

Leaked caps cause more noise eventually leading to bus glitches, especially when a 'beefier' CPU pulls more load.

[Zac67]

Then you can rule out the caps, I suppose. Have you checked the PSU voltages and possibly ripple?

[Zac67]

Another thought: the standoffs need to be exactly the correct height - if the CPU board isn't exactly parallel the contacts can be bad.

You can measure DC ripple by setting your multimeter to AC voltage, the lowest setting available (2V or lower). For somewhat reliable values you'll need a true-RMS meter but you should be able to get an idea with anything sensitive enough.