[ODE] strange oscillations while steering a roboter arm

[Sebastian Gieselmann]

Hi
I'm trying to to implement a PID-Controller, for steering a roboter-arm.
For beginning I only want to lift the arm.
But there is a strange behavior.
When I increase the force (force=force+0.1 in every simulation step) for
the HingeJoint, connecting the arm to the body, there are some strange
oscillations.
The arm moves up, but with different speed. sometimes it nearly stops
moving and moves on afterwards.
This increasing of the force is just a test, but this behavior appears
with the PID-Controller, too.

Can somebody help me??
I've got a python script, which shows this problem. So tell me if
someone is interested in it.

Tanks for helping