I tried deleting preferences also. Did’nt work out. I contacted JVC and they told me the GY-HD100 needs a firmware update. If you experience the same problem, contact your dealer or JVC in your country and ask for the need of firmware update.
Other advice i’ve got is using good (jvc) hdv-tapes. I used common sony dvm60pr3. Didn’t test with hdv-tapes yet but it is worth trying.