Try updating the firmware.
Use this one for North American machines.
This one for Europe if it has the Postscript kit
And this one in Europe if it does not have the Postscript kit.
The Postscript kit is an actual thing, if you load the firmware thinking you will get the PS kit free you would be sorely mistaken, things will go poorly.
Honestly, I don't know. It will probably require a service call to Xerox, have a tech wipe the machine to eliminate any possibility of corruption.
As for guesses, it could be something on the LAN waking it by polling it routinely, you could disable SNMP to prove/disprove this, but in any case, it may go to sleep, and it would tell you exactly why, but it would not tell you what was polling it, so you wouldn't know what to do to stop it.
It could also be the battery (CMOS Battery) in the machine, this would affect the time on the MFP, so realistically, if the time is right, it isn't that, if the time is wrong (way wrong as well as the date) this could be causing the issue, but it doesn't explain why a firmware update would have temporarily resolved it.
maybe there is NVM setting wrong?
.How to Disable Power Saver
To disable Power Saver do the following:
Change NVM location 700-142 = 0. This will disable Low Power Mode
Change NVM location 700-131 = 0. This will disable Sleep Mode
nvm settings can be changed by xerox engineers