[help]about VCO centre frequency unstable

recently I am measureing a CMOS integrated VCO corss couple topology
but I find the centre frequency is unstable
about 100KHz @400Mhz
can anyone tell me what cause the frequency unstable and how to reduce the unstabe freqency

The free running frequency of the cross-coupled oscillator depends mostly by the current flowing through the coupling transistors. Also the unknown parasitics of LC tank can affect the stability.