[jjance]

Here is some information I came across on the Curtis Controller site. It may or may not apply in your case

FYI

M1 Field Min 1 – 10 A
Defines the minimum allowed current in the motor’s field winding when the vehicle is
traveling in the forward direction, operating in Mode 1. The forward Field Min setting
may affect the vehicle’s maximum speed and, to some extent, the smoothness with
which the vehicle starts and transitions between drive and regen. If the parameter
is set too high, the vehicle’s top speed will be reduced and torque bumps may be
evident when the vehicle is inched or changes direction

I like torque bumps better than shuttering :) Since you just replaced the controller this might be the problem.