If it was mine, I'd probably start with some basics.
If you run these as a set a-b-a, seperate them, and see if you can tell what loco is causing the problem, eliminate the b unit first.
Run them in the oposite direction, do they do this no matter which loco is first, and what direction they are going.
Check the wheel spacing, on the unpowered unit as well, maybe one is just a tad tight, and it causes binding going into a curve.
At the same time, check your track spacing in the curves.
Take the shell off, and watch ever so closely, and listen, as maybe it's a drive shaft or worm gear problem. If the thrust washer is worn on the worm gear, you could get binding.
Since you do regular maintainence, double check the gear towers for any old grease, and make sure each truck can swing freely.
Mike.