Hi guys!
This is my first post here so please pardon mistakes etc.
I have a device which can measure the resistance of a conductor using 3-wire measurement.
To check that is works correctly we need a some kind of resistance imitator.
For simplicity let's say that the device drives 1mA current into this imitator and measure the voltage across it's terminals.
Then it calculates the resistance dividing voltage into 1 mA.
I have developed this schematic as a resistor imitator:

But when I power up this schematic (and NOT pushing any measure current between OUT and ground) I got this:

Zoom in:

So the gate voltage increases over time until it reaches around 4.2V. Then we can see some oscillation.
This process repeats with some more or less stable period.
Maybe someone can explain why gate voltage changes this way?
If you need some extra info about the schematic etc please ask.