My Wabash 4-6-2 #672 was like that when I ran it for the first few times since 1986 or so. It had only been tested to make sure it ran before I got it this year. Every night I ran it, it got better. And took less throttle. Oiled it once a week for 4 weeks. Now it even takes less throttle. Just crank up the voltage and make sure it is oiled! Hope that works for you!
Edit. Mine would stop in 0-36 curves also. Now it just glides on through!