different results of simulating mechanical resonant frequency

I am trying to simulate the longitudinal (x axis) mechanical resonant frequency of a free standing beam. I set the constrain at y and z axis and did the simulation. However, even I keep every parameters and mashing same, the enginfrequency changes a little bit every time I re-solve the problem. Shouldn't the calculation results be same?

I also add a layer materials with different mass, and simulate the frequency change. However, after I add the layer, it always shows error message: "out of memory during assembly". I don't know how to solve the problem.

I am using COMSOL 3.4.

Please give my some help.