usually indicates the 300VDC bus inside has gone bad. Truthfully, when I used to run in to those problems on those drives i did one of two things.
Gave the customer the option of getting an exchange drive, or have the local Fanuc office come in. Not that i couldn't fix drives, but I didn't want to have to warranty something like that.
My guess is if the transistors are good, you have a failed CAP in the DC link.