imu data (heading, pitch & heel) with intermittent FALSE values

Hi again,
when running my first real life tests, I observed that OpenCPN’s MAG Hdg, Pitch and Heel are toggling between zero and the real values in more or less regular intervals most of the time. Diagnostic data as shown by OpenPlotter reveal many HDM and XDR sentences holding FALSE values.
example:
$APHDM,250.127,M*30
$APXDR,A,-5.878,D,ROLL*46
$APXDR,A,0.799,D,PTCH*7C
$APHDM,250.127,M*30
$APXDR,A,-5.878,D,ROLL*46
$APXDR,A,0.799,D,PTCH*7C
$APHDM,False,M*40
$APXDR,A,False,D,ROLL*1A
$APXDR,A,False,D,PTCH*08
$APHDM,False,M*40
$APXDR,A,False,D,ROLL*1A
$APXDR,A,False,D,PTCH*08
$APHDM,250.38,M*0F
$APXDR,A,-5.983,D,ROLL*43
$APXDR,A,0.779,D,PTCH*72
$APHDM,250.38,M*0F
$APXDR,A,-5.983,D,ROLL*43
$APXDR,A,0.779,D,PTCH*72
$APHDM,250.413,M*32
$APXDR,A,-5.979,D,ROLL*46
$APXDR,A,0.777,D,PTCH*7C
$APHDM,250.413,M*32
$APXDR,A,-5.979,D,ROLL*46
$APXDR,A,0.777,D,PTCH*7C
$APHDM,False,M*40
$APXDR,A,False,D,ROLL*1A
$APXDR,A,False,D,PTCH*08
$APHDM,False,M*40
$APXDR,A,False,D,ROLL*1A
$APXDR,A,False,D,PTCH*08
$APHDM,250.582,M*3B
$APXDR,A,-5.975,D,ROLL*4A
$APXDR,A,0.8,D,PTCH*73
Any idea what’s wrong here? Can you suggest an approach to further analyze this issue?
best regards,
Michael