Did you got any Waring like "Large Displacement"?. If you got so kindly go to study properties and select Soft spring to stabilize the model option to Stabilize the model by applying external stiffness.Then you can find that where your boundary condition or contact or loading not properly applied. Then correct that one and remove soft spring option and run the simulation. Else post your model using pack and go. we will look at your problem in depth. Thanks
A D Kumar
EGS Computers India Pvt Ltd
This is my first guess as well.
other option is applying temp loads to faces