Apparently he was diagnosed and suffered from bi-polar disorder.
"They've got me on this new medication. Lithium. I'm a manic-depressive. Sometimes I think I'm fucking Superman and sometimes I can't get out of bed. It's wild."

and

"This is the Lithium...this is so I don't go fucking nuts and start handing out hundred-dollar bills outside the Midtown Tunnel."

Source (a book written by two FBI agents who worked on the Gambino squad).:

Boss of Bosses: The Fall of the Godfather : the FBI and Paul Castellano