Maybe it makes sense that low PID values result to unstable behavior. The position of the rotary joints are given in radians (probably between -3.14 and 3.14 or so) and the velocity is radians per second, which might be quite large.

Let's say you have the joint in initial position at angle of 0 radians and want to move to the opposite side. So the error will be some 3.14 radians. And to achieve that position in less than a second, you need to move more that 3 radians per second. Taking just the P component of the PID regulator, you want to assign a velocity of kp*error = 0.001*3.14 to the joint. That won't do.

You should not aim for small PID values. You should aim for PID values that make sense with your application.