Hello,
it's never a good idea to blindly change components on suspicion, especially not on the 3458A.
The nvRAMs very probably have nothing to do with this error.
Check their Date Codes, please. As well, provide Date of Production (DC on other components) for your instrument, so that we have the possibility to judge possible errors.
This kind of error often relates to the fast comparators EL2018 on different boards, in this case I assume U142, or U181, less probable U405 on A3 ADC board.
There's another of these comparators on the AC board.
You get much better ideas what might be wrong on xdevs.com, please check his different repair articles (3 or 4?).
At first, please make regular measurements in all modes, which fail and which work.
Short NPLC and longer NPLC in DCV might have different behavior and might indicate a failure of said EL2028.
You did not provide any details, how and when you got the instrument, if it was DOA, or failed after transport, or some usage time.
If latter cases apply (vibrations?) then you should check all of these serial fiber optics interfaces, if they are properly stuck into their counterpart.
Sometimes, some of these optical interfaces fail.
Please make systematic failure analysis on this instrument, so not to increase your trouble.
Frank