Linkage wants to "hang up" and stick at a higher rpm...Only way I could get it back at normal RPMs...It's the original setup when I bought the car...I like the Jegs setup...I like this style. Mounts to the intake. Doesn't move around like the one that mounts on one stud.
Why are you using a large return spring?
https://www.jegs.com/i/JEGS/555/157336/10002/-1