In my experience of these types of problems, the source is more often at the front of the aircraft.
Check if the flight deck glareshield or instrument panel is made of a metal composite or covered in metalized paint. If so then these require good bonding as the static from the windscreen can be transmitted to the instrument panel and thence the instruments. Check for corrosion under the bonding strips.
Obviously, check the windscreen bonding which probably ties together the two main panels with the fuselage and perhaps should tie in with the glareshield / instrument panels. A check of the windscreen heating circuits might also be worth doing.
The left handedness of the problem might be a clue; IIRC the 125 wiring scheme separates the digital buses between left and right side of the aircraft to provide some failure proof segregation.
A more obscure fault might be with the bonding of the main entry door. As implausible as this sounds, the door, located on left hand side, completes the fuselage’s electrical cage. As an example of electrical interference, some older HF installations would ‘ring the fire bell’ or disturb the OAT gauge when transmitting on the ground with the door open, but otherwise were OK in flight.