430 and King KAP150 in Australia

I have a Mooney 201 with Garmin 430 and King KAP150 autopilot which
both seem to work OK individually but when I couple the Autopilot
using the NAV button it works OK for the first 10 seconds and the nav
commands release and the aircraft slowly tracts in a large circle ( if
I let it).If I hit the NAV button again it gets the nav commands from
the Garmin again and then releases after about 10 seconds again.
The other problem which seems to be related to this is The HDC button
does not work at all on the Autopilot,but the BUG command from the CDI
does feed correction signal to the Autopilot during NAV command(with
the GPS tracking signal).
The HDG and NAV lights seem to operate as I expect them to on the
Autopilot.
This is my 4th Mooney so I am reasonably in command of its operation.