How steep is your grade? Honestly I prefer a little speed variation, as it confers a "natural" feel. Sometimes if the speed control is too tight and reactive, it doesn't feel "train-like." Instead it feels like you're controlling a robot!
Did you try it with cruise control turned off? I don't believe that removing the tires, adding springs, spacers, etc., will resolve this issue. If your grade is excessively steep and the trains are very heavy, it might be outside the range of scenarios the manufacturers envisioned when they designed the system. Back in the conventional control days, the solution was to put a voltage-dropping resistor on the downgrade. Maybe- just maybe- you could simulate this by sending a reduce speed command using Lionel's IR sensor track. Not sure how you could do this with DCS.
