viscoplasticity

I am tyring to solve simple shear problem using a large deformation FE code based on the framework developed by Fleck and Hutchinson for gradient plasticity/viscoplasticity problem (see the attached paper). As the per the formulation, the independent variables are the displacement increments and eqv. plastic strain rate increments. Two sets of equations are solved, one for displacements and the other for plastic strain rates at nodes, following the the above formulation.

I am using my own fortran code to solve a large strain 2D elastic problem using FEM in Updated Lagrangian framework and I am using incremental formulation. It works well for 2D pull of a square block with 2 elements and I've compared the results with a commercial FEA package. But while solving bending of a cantilever beam the results starts to blow up and the elements distort abruptly after certain time step. This happens only when the load or the displacement given at the free end of the beam is large otherwise it works fine.