I agree that your father is very good when he is normal. Alcohol alters behaviour as natural inhibitions are lost when your father consume alcohol. So people under alcohol behave in a manner which they normally don't do. The amount and pattern of taking alcohol varies individual to individual
only can understand what makes your father to consume alcohol. He may try Psychotherapy
and if possible de-addiction. If your father volunteer himself for de-addiction the success rate of de-addiction is very high and the cooperation of every family member is the main role of treatment. I pray god for your father to come out of alcohol as early as possible.