Oscillations during processing

DAK565656

Such a fundamental problem, but I didn't see disquisition. Solving absolutely steady problem of external flow I achieve sometimes residual oscillations for pressure and components of velocity. Maybe the reason is wrong timestep, but how can this timestep influence on steady problem? Changing properties of mesh may result in convergent or divergent solution. Changing the thickness of BL for only 1 mm. from convergent solution thickness can lead to divergent solution. This results are very sad. This means that I have to use different mesh properties for each geometry (for each angle of attack in my case) that don't lead to divergence...