I took a lok at what you have and I can see several problems.
You have part of the answer right in front of you. When you jog the motors are going full speed and you said if you set the velocity below 18ipm they get choppy. When you run the g-code they are running well below 18ipm I am assuming.
Try changing the step setting to 1/8 or possible even 1/16. Large step settings will give you chopyness at low speeds.
Those motors are rated at 83v max and at 24v you are sacrificing a lot of speed. Info from the keling website 6.8mh inductance = 83v max.
At 24v you may be disappointed in their performance.
The amp rating is 2.8, don't set the drivers any higher or you will cook a motor or 2.
Verify that you have the motors wired in bipolar parallel. See kelings diagram if you are not sure what this means. If the motors are 4 wire then disregard.
Try all that and let us know how you make out.