Based on your very good description of the problem, I would say that the drive is very sensitive to over voltage.
The easiest way to correct this is to add a buck transformer to the 240 V line on the input to the RPC. Drop the incoming voltage to 220V or so.
Interestingly, I did not experience this problem on my OT equipped lathe (1989 vintage) and the input voltage was 245 - 250 on the input line at the time, the power company has since corrected this. I don't remember what drives were in the machine. All 3 legs of my RPC output were generally within 1 volt of each other, under load, and pretty much mirrored the line input voltage.
Another way to fix this is to do what I did and rip out everything that said Fanuc on it (except the spindle motor) and replace it with modern hardware. I have a low tolerance for antique controls.