Did you try a factory reset?
Google "marantz sr5003 factory reset". You don't really have to find it for your specific model, most of the Marantz's from the same period all use the same key combinations to reset.
It may not fix your issue, but just like rebooting a computer it is a good thing to try before you move on to more expensive options.
However, you will need to rerun Audyssey and reconfigure any settings that you have made such as assigning inputs, etcetera.