You can get a bad cable from anybody on occasion, but it's odd that it was only intermittently bad.

If it comes back, see pages 18 and 23 of the guide to start with. If you set the resolution under "Input/Output Assign | Monitor Out" to "Source" then you can set the desired resolution for each input separately under "Source Setup | Picture Adjust". You could also set the "Input/Output Assign | Monitor Out" resolution to 1080p - in which case I think the "Source Setup | Picture Adjust" resolution doesn't need to be set because that resolution will be applied globally.