my VAN'S RV6 is equipped with a fresh updated FUNKE TRT800H Transponder and a Garmin 496 GPS sends the ADS-B Data to the Transponder.

I can see my flights very well in FlightRadar24, altitude is okay, but my flying speed is wrong. My real speed is between 110 and 150 Knots but Flightradar History shows wrong Speeds between 110 and 515 Knots ????
Is it a Transponder Problem ? Last Year I had no ADS-B out and the plane was sometimes visible with MLAT, but with correct speed.

Search for PH-CRJ in flightradar24. (I am not allowed to post Link)

Any Idea ??
Thanks, Achim