  Three-dimensional magnetohydrodynamical simulations of strongly magnetized
``light'' conical jets have been performed. An investigation of the transition
from sub-Alfv\'enic to super-Alfv\'enic flow has been made for nearly poloidal
and for helical magnetic fields. The jets are stable to asymmetric modes of jet
distortion provided they are sub-Alfv\'enic over most of their interior but
destabilize rapidly when they become on average super-Alfv\'enic. The jets are
precessed at the origin and the resulting small amplitude azimuthal motion is
communicated down the jet to the Alfv\'en point where it couples to a slowly
moving and rapidly growing helical twist. Significant jet rotation can
contribute to destabilization via increase in the velocity shear between the
jet and the external medium. Destabilization is accompanied by significant mass
entrainment and the jets slow down significantly as denser external material is
entrained. Synchrotron intensity images satisfactorily reveal large scale
helical structures but have trouble distinguishing a large amplitude elliptical
jet distortion that appears as an apparent pinching in an intensity image.
Smaller scale jet distortions are not clearly revealed in intensity images,
largely as a result of the relatively small total pressure variations that
accompany destabilization and growing distortions. Fractional polarization is
high as a result of the strong ordered magnetic fields except where the
intensity image suggests cancellation of polarization vectors by integration
through twisted structures.
