Try using an antenna mounted on top of the airplane if you are trying
to use some type of internal antenna you ARE at going to have
problems.

With the antenna located INSIDE the airframe you have 'wave guide'
conditions allowing all types of radio RFI that extends well into the
Microwave region exceeding the GPS signals

Don't be cheap and Stop messing around ! have an external antenna
mounted they cost about $ 187 ( RAMI AV -GPS [aircraft spruce] )
takes about an hour to install. Route the External GPS Antenna cable
to a BNC bulk head antenna connector located on the instrument panel
then a short patch cord to the '396

If the airplane is a rental tell the owner to have one installed. It is
NOT unreasonable in this day and age of GPS portable navigation
devices that Rental aircraft provide the external antenna.