The HDMI output setting on my Panasonic player seems to change occasionally from 'Enhanced' (deeper blacks and colour apparently) to 'Standard' most of the time (the picture will change to Auto instead of 1080p) and I'm not aware of doing anything to make it to do that?
Does something similar happen with anyone else - could it be the cable? Its connected to a surround sound amp.

No it couldn't be the cable.

Thank you I'll look into the settings.