There is some corrosion in that wire. Why would this affect the one auger motor and not the others if this is the problem?
when the electrician was there did he check the voltage under load,,,and with a clamp on amp meter?
if so was it high on one leg?
motors of the same rateing doing differant jobs can require more (or less) amperage. and react differant...
but if others seem to work fine from the same electric source. it may not be the source...quickest check of the source is to put a bigger load on it.. (although not the safest)............!
I would start testing the motor...
if you have an ohm meter check the motor windings with the motor disconnected,....(be sure to label them before disconnecting them)
..they should only read a few ohms but be equal...not a dead short or real high....
if the motor windings check ok ...
it could still be the Motor,,,
check each lead to motor casing (ground ) ,,,They should read infinity on the highest ohm scale..( if not infinity and all are consistant of near same reading it is likely moisture in motor box) .....
*Make sure You use the Motor Casing as some moters have Isolation mounts (rubber) that seperates them from ground..