DC motor is pulsing when connected to power supply
Without seeing the motor specs I expect the servo can draw 10 x the rated load current when accelerating to reach full speed. Your supply can only support a slow ramp.  This is normal for good DC motors. The current drops as rapid as the speed rises as current is a force that controls acceleration then rated load at rated RPM. Surge currents are always much higher than the static current rating.

This means the supply cuts out from overcurrent causing shutdown then recovery, so you get a jerky pulsed acceleration.

Your only options are ; use a slow ramp for speed up, get a bigger supply.  SLA Battery option may be possible to support the surge.