First, try to reset iDRAC using racadm racreset from the terminal using root administrator privileges.
Try to do an iDRAC Reset to Default using any of the following:
When you replaced the idrac on your r710 did you replace the express card that is located under the raid controller or did you replace the idrac enterprise which has the external NIC port? The express is the piece that has all the idrac logic built into it and that's the piece that will need to be replaced. I've also found from experience that sometimes it can take several restarts and power cycles before the idrac and board begin communicating properly again.
All that being said if you've replaced the idrac express and it's still giving you idrac communication errors it is possible the port which the express plugs into on the mainboard itself can have gone bad which will require the entire mainboard being replaced.