Intermittent starting issue - 2007 Impreza 2.5i

[Notamechanic]

I have had an ongoing issue with my 2007 Impreza 2.5i and have struggled to get a diagnosis. The car will start perfectly fine in the morning, but after driving around, will not start unless it has been sitting for a long time (2+ hours). When I turn the key, the lights and radio come on, and there is a brief clicking sound and then nothing. I have replaced the battery and the starter, and the battery terminals are clean. I replaced the starter relay and the problem went away for a few months, but this issue has popped up again, and I am seeking any insight as to what could be the cause.

 

[bmart]

Voltage on your battery with car off after car has been sitting a whole day, then again at idle, then again at RPM?