Automatic trunk lid

[w12a3d]

My car have the automatic/hydraulic trunk lid. It's work but not as it should be. The problem is when it's cold outside the trunk take a long time to open, sometimes I have to pres the button twice. Now in summer, it opens at first but after few seconds it starts to fall or closing by itself (pump doesn't running). I tought that is because pressure drop. But when the lid is fully open stays there and after few seconds like the valve opens and the lid slowly starts to go down. So, I have checked the position sensor and it's working. I don't know what's wrong.

Also, if the lid is closing I press the button again, the lid stops there and after one second starts to drop itself. Is there any calibration procedure or something like that?