Hi, jayanthi:
I checked your structure and results. I can see there are many up and down in the s-parameters. You may consider it as instability. In fact, I believe it is not. It should be true resonances caused by the finite ground plane and the 2 monople like structures in the finite ground plane. Basically, they are creating many parasitic resonances over the wide frequency bandwidth you are interested in. IE3D is capaturing all of them in the simulation.
Best regards,