Elliot,
I find this a very interesting topic........ hope the remedy is located in a short period. You stated the third power supply on the South panel is contiguous with districts from the North panel, correct? Just a thought, but with this arrangement between panels, what about any back feed of power between the power supplies of South/North panels and related trackage ? How much assist would it be should you meter the power feeds from the various power supply circuits and check for spikes, occilations, etc. ?
Just thinking...... sometimes a dangerous thing...!!
Jesse