If the issue has remained between CPUs, and you have performed some harsh memory tests with that ram in the X220, it’s likely a cpu pin got bent/dirty/etc during your switch. Especially if it started after switching to the i7.
The memory could also be on the verge of failure that the x220 is not seeing, I would make sure it passes all the memory tests you can throw at it. still not guaranteed its not operating just enough out of spec for the 440 to dislike it.
Its a toss up, but I would lean towards the mobo if it started immediately after the swap. But a small ram stick is super cheap and easy to fully eliminate that before going further.