I have seen this on delta primary feeding a transformer with a delta secondary, and the MGN was lost down the road up on a pole, if the transformer has a primary winding starting to leak to case it can cause the MGN to rise in voltage and can be dangerous as there is no fault current path back to the zig-zag grounding transformers used by the utility to derive a neutral that is bonded to the MGN, I would get the utility involved but expect them to not find anything if it's not happening when they come out,
it took me several times to get it to happen when they were there, the case above happened after a lightning strike that both cause a connection on the MGN to start failing and a primary winding in the transformer bank to intermittently start leaking, they first found the bad MGN connection but after fixing this, the next time the winding went into a case fault it tripped the primary fuse so they had to replace the transformer tank that was bad.
Check your system grounding to electrodes and very important if you have a city supplied water system with metal piping that the bond to it is good, as this can limit the voltage from the primary if such a fault occurs, as a fault like this can cause the system grounding to go as high as the primary if there is no fault path and can be very dangerous.
Also many DVM's will read high frequency signals as voltage error's so make sure your not getting HF being imposed on your 480 volt system from a HF source (such as VFD's), try using an analog meter when it happens.