The DALLAS DS1220Y is the same one used in the 3458A.
'quarks' also had a 1989 vintage NVRAM in his instrument, and mine was 14 years old, both still working.
But other 3458A owners reported data loss after 10 years already.
So you should get a new NVRAM.
We have copied the contents of our NVRAMs to fresh ones, they are still available from maxim, or from mouser for a low price.
The new designation is: DS1220AD-150+, around 8$.
As you already have saved the content and are able to write it back, you could check, if the NVRAM actually contains the default calibration constants only.
Maybe somebody before experimented with the loading process, or there had been a checksum error before, and the instrument loaded the default values by itself.
Anyhow, this instrument needs a fresh calibration, so I look forward to a calibration video.
Frank