Phase Noise & tstab

Hi Guys:

I find that in my PNOISE analysis in determining Phase Noise for an CMOS LC-QVCO, the phase noise plot changes sginificantly, when I change the Additional Time for Stabilization (tstab) in the PSS form. Can anyone tell me why this happens and how must one select tstab for accurate Phase noise simulation. Thanks in advance

Rgds