How to make the frequency stable in the capacitive sensor using 555 timer?

[Matinda]

I'm trying to build a capacitive sensor using 555 timer.
The idea is to measure the frequency output of the timer which depends on the variable capacitance.

The problem is that the frequency is unstable despite the same fixed dielectric is put between the capacitor plates. Here is my schematics :

Any help to solve this issue is highly appreciated.

Thanks!