@Thrane Hi, which sensor are you using? Some of those airspeed sensors are quite sensitive to temperature, it's then actually a good process to relcalibrate the sensor go get accurate airspeed readings.
If the temperature doesn't change between bootups, then it might be something else.
I used a SDP3x from Drotek for quite some time and this one has internal temperature compensation and never requires to recalibrate. I would recommend you to get one of those if possible; you will also benefit for accurate airspeed readings at low speed.