We don't currently have a Phantom Omni here, so I can't test your code. But your angular spring constant seems quite high. Try using the following values to see if they produce better results:
Code:
spring = cyl.addSpring(viz.LINK_POS|viz.LINK_ORI,linearKd=10,linearKs=1000,angularKd=1,angularKs=10)