Oscar Kilo
You should use an external GPS antenna (aerial) - even if it is only the one which attaches with a suction cup to the unside top of the windscreen. The built-in antennas on Garmins are often marginal in an all-metal plane.
I have twice had GPS signal loss, lasting about 20 secs, in a very specific place in the South East, with a Bendix/King handheld. Presumably there is a powerful transmitter there which saturates the GPS receiver. But never seen it with a KLN94 with a proper rooftop antenna.
When you look at the ease with which you can become "uncertain of position", a GPS is far more reliable than any other way of navigating.