I want to add a Keep Alive to an N scale Spectrum 2-6-6-2 DCC on board. Where do I attach the wires?

I have a first generation Spectrum N scale 2-8-0 that runs like a Swiss watch on the level and uphill. But going downhill with ten cars and a caboose behind it, it has a very jerky motion that must be a bind because it is in sync with each revolution of the wheels. When I put it on Bachrus rollers or upside down in a cradle, there is no bind. I checked the clearances in the rods and valve gear, and they appear to be OK. The fact that this only happens when going downhill, especially on my steeper (3.85%) grades has me stumped.