IRN75H-CC VSD Fault 41. What is causing this?

I have an Ingersol Rand model#IRN75H-CC air compressor and I am getting a VSD fault 41, my maintenance techs have been through it thoroughly and a certified air compressor service company has been through it thoroughly. Here is the service tech's notes:
{Unit was in auto standby when i arrived. Put a load on the unit, loaded unit at both full load, minimum load and modulation. Unit did not trip
after an hour of fluctuation load. Performed a interface board test (passed) performed a power module test(passed),
checked air gap around stator and rotor had at least .020 clearance all the way around, performed a diode test
(passed), visually inspected Halifax sensors no signs of failures, megged motor(had normal results), performed a
milli-ohm test and had 22-23 milli-ohms between the legs within the range for a 100 degree motor. Reassembled drive
and motor. Put unit on line if problem continues recommend replacing the output module on unit. Completed check
sheet. While taking reading found input and dc buss voltage are fluctuating drastically on the controller but are not
actually fluctuating.}
IR suggested the output module is bad and needs replaced. Now I read some older discussions from 2006 and 2010 and people suggested that the incoming power was dirty. Anyone have input on this?