J3 Jittering

I've just completed the construction of my AR4-MK3. Upon testing, I've discovered that J3 is not working. Everything else is working great, and it's difficult to hold in my excitement! But J3, when I jog it, jitters. It just somewhat shakes. Sometimes I get the "J3 Collision or Motor Error" log. When I run J3's calibration, it jitters for the first half, but moves smoothly during the second half (weird...). I also ran the encoders test and all seem to be working, including J3's encoder.

It would seem J5's limit switch is not working, but I imagine that would not affect J3. J3's limit switch is in fact working. Additionally, when powered off, I can manually move J3 very smoothly and easily. No mechanical friction that would prevent a motor from turning.