Is the rear-most unit of your A-B-A set powered, or a dummy? I'm assuming you have a 4-motor set and the rear unit is powered.
Going downhill at slow speeds below 8 scale mph, what you might be experiencing is called "bucking." This happens because the worm gear drive used in MTH diesels is self-locking. The wheels can't turn the motor, and the weight of the train cannot "push" the loco downhill. When it tries to push, the worm wheel in the last truck block locks up against the worm on the motor shaft.
At the same time, the speed control may detect the slight increase in speed, and begins to rein in the voltage to ALL the motors. If the timing isn't exactly right, it creates a vicious cycle and you end up with a surging effect. I've seen this happen, even on a very gentle downgrade, with my own locos.
Lionel's Legacy diesels have back-drivable gears (as did all Postwar diesels.) So this kind of bucking can't happen. In your case, you may not be able to fully resolve the problem. There are some things you could try:
(1) To test my theory, repeat your scenario with speed control turned OFF (you can temporarily disable it with your DCS remote.)
(2) If possible, put the motor with the tach sensor at the rear of the lashup, adjacent to the first car of the train
(3) Take the hoods off your locos, and make sure there is no mechanical resistance or binding in any of your motors or power trucks. You can check this by turning the flywheels slowly by hand. You might have to turn each one several times for all of the gears to run through their cycle. Any sticking or binding can cause the problem you described.
(4) And finally... A radical motor-ectomy. Remove ALL of the motors except the one with the tach sensor and stripes. This may eliminate the bucking. It will run smoother and slower on flat track because the other motor(s) are no longer fighting the one with the sensor. Of course, a single unit with half its weight on unpowered axles may no longer have enough traction to pull the train UP your grade.
Please try the first three suggestions, and let us know what you find.
