Emco 320: Supply Power or Servo Amp issue?

I finally have the RPC hooked up to the EMCO 320. Machine functions. EXCEPT it gets both server errors 414 & 424, with the DC over current bit set (0000100). Alarm on the servo amp gives the DC light. These errors don't happen right away; but w/in 5 mins or so while idling. This is what I know:

1. Supply Power comes from an RPC (AD20 digital). It's somewhat balanced (digital) however, the input voltage on one leg is 5% too high. e.g. 240/240/253V. Translates to 380/380/394ish. When I have higher voltage (400/400/420), using the 380 outputs of the autotransformer instead of the -5% outputs, the servo amp fails right away. I balanced the RPC (per the manufacturer) w/ RPC of 245/250/250 (don't know why my L1/L2 input power went from 240v->245v; was also another day), but this made the amplifier errors occur much quicker (1-2min).
2. If running at the lowest voltage configuration (240/240/253)V and -5% autotransformer outputs, AND the spindle is turned ON, thus putting a larger load on the system and RPC, the server amp doesn't fail (we're not cutting anything; just putting a larger load vs just idling).
3. Gone though the checks on the servo amp; everything is connected correctly, using the INTERNAL dissipation resister.
4. Servo amp board: A16B-1200-0800/09b, base board might be A06B-6058-H221 (not around the system right now). Fanuc 0T system

Question: Is the EMCO 320 (or rather the Fanuc servo amp) pretty finicky when it comes to power, or SHOULD you be able to through a varied range of power to it and have it work correctly? The documentation says supply voltage of 380V. The metal sticker on the outside says 400V. When I had a better balance config, the input voltage was around 390V +/-3-5 volts; but the amp failed quicker in idle.
If it's not that finicky, then I would assume (ya, never assume anything) that it's likely the servo amplifier. Anyone have any experience on this: top board? base board? failing Q1 transistor?

I would love to drop the power down 5%, but I'm at the limits of the autotransformer (has 230V input, but the RPC output is in the 240+V range).

I know this might be a special, issue; maybe someone's experienced this before. Thanks!

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.

Update: We haven't run it very much to ensure it's 'fixed' (other priorities), but I switched the 3-phase legs around to see if the higher voltage leg might work better on another phase, and lo and behold it seems to have! We have run it for a 2-3 hours straight without it failing. TBD...

...Which brings up another question: Do the servo drives (control circuitry, at least) use a single phase? This was my hypothesis and maybe this explains it? I thought the x/y/z servos were still 3 phase but maybe the driver is not? I just don't have that much experience with them. Thanks.

Yes, the control side is single phase.