Consistency is the bedrock of effective execution. Be wary of people who are sometimes amazing, but often not dependable or consistent in their behavior.
They are analogous to an abusive relationship. They give you just enough hope that things will improve and then they let you down.
The amount of time and energy you spend trying to manage their feelings and their behavior can be better spent replacing them with someone more consistent (and therefore more effective).
At best these people are unreliable and distracting, at worst they are toxic and dangerous.
Further, getting rid of them can actually boost team morale because colleagues see that bad behavior is not tolerated and their own consistent efforts are recognized and valued.
Make the hard decision and move on quickly.