I sometimes record a flight with an actioncam, I synchronize the airdata (derived from the GPX log recorded by SD) with the video in Dashware.


The heading and altitude information are very accurate, but the speed is very unstable resulting in running up and down the speed tape on the left side of the videoscreen, see the analysis below:

I have recorded many log by means of different iPads and an Android Smartphone, but the result is always the same.
Is something going wrong during the conversion process from SD to GPX format? During the flight I do not notice a fluctuating airspeed on SD.

If there were a conversion error I would expect it to be consistently wrong. The values recorded into your flight log are taken every 5 seconds, and represent what your GPS is saying the speed is at that instant. I wouldn't expect a fluctuating value if you're not seeing one onscreen.