I agree with Mixture. Some times it is the source.
One of our Samsung tv's does what your Pana does, about 1 in 10 times.
I have no idea why it does it, as the cat doesn't watch the tv.
The only solutions that work when it is misbehaving are
1. Turn TV on and off. (Helpdesk 101)
2. Keep cycling through the options,and eventually it finds the "proper" 16.9.