Why does my dad suddenly change his attitude out of nowhere?

well i started talking to my "real biological" father the last couple months. My mother kind of sheltered me from him until i was about 21 and always told me he was crazy. i suspected because of child support reasons but i really dont know. i am now 24 and finally got the courage to engage with him. Our relationship is very odd and it leaves me lost. We have now gotten to know each other a bit and are past the "nervous" or "strange" stage. He just acts plain crazy sometimes. One day we will be out for ice cream and having fun and talking and everything will go smooth. Than i notice about once a week he just goes crazy. He will call me up threatening to take me to court because i left my truck at his house (as ive been doing this the last month because i have been working on his farm). He will just become nasty and mean out of nowhere. He will tell me that i should not come over anymore otherwise the state police will have me in cuffs but than he will call me the next day to come over and eat dinner? I suspect that when he gets into a fight with his new wife (my stepmother). Than he has a bad attitude and takes it out on me. But to this extreme makes me really wonder that my mother was right about him being crazy. Is this some kind of psychological disorder. If so can someone please explain to me?