about capacitor measurement

Why is the Q-factor of capaitor not stable when I'm measuring
capacitor by Agilent measurement equipment ?

Modern, Hi Freq, small capacitors have Hi Q (low ESR).
Often measuring jigs have higher resistance.
The calibration of the jig correct sistematic errors, but often a part of the resistance is involved on the contact and may be reduced and made stable only with the manual job of a higly skilled and expert operator.
Surfaces should be clean, free of oxidation, contact should be zero-lenght, be free from resistive metal/alloy (like sn-pb) etc.
The use of an VNA give higth uncertainty because the observed value is near to the external circle of the Smith Chart. The old Q-meter is better than VNA.

I don't think so.According my experiences,VNA can give more stable results than some general LCR
equipements do.But you should take care of your model of capacitor.