To re-iterate, I'd suggest starting by improving the outside rail conductivity (lessen the resistance) at the track joints by either tightening the rail connection around the pin (pliers or crimpers) or as Bigkid mentioned by soldering the joints at the problem area.
If that doesn't improve operation, then identify where the Outside rail connection back to your Track Common is located in relation to the problem area. If you do add another Common connection to the outside rail, you'd need to keep DCS track drop numbers and location recommendations in mind.
