Hi,
In principle, there are many possibilities some of them related to the step size and some of them related to the system itself.
so just change the step size (temporal or special). and check the convergence of the program.
if it is not converged, so please check your system design first. It might have no solution and the system conditions give chaos.
In this case, start with well know system (studied before), then modify it step by step toward your system till you catch the malfunction position in your system.
hope that helps,
thanks
Mohamed Abouseif