Please help 3 year old Chihuahua is a TERROR!!?

I inherited my sisters 3 yr old Chihuahua...he's already past puppyhood. I've been told that he needs to be socialized more by people in the pet industry. However he is getting better. Since I have him now for time being, there are times when he will viciously bite me to the point I've now got nerve damage in my hand and scarred horribly, when he bit me I should have got stitches but did not. I live in Dallas TX now and my sister is still in Nashville TN so I'm the person to discipline for time being. However this is the same dog that will let me pet him and he cries for me and even licks my hand. However sometimes he gets some wild hair and wants to violently hurt me. I'm desperate for suggestions please, I know that I can not give him to someone else, however he has shown much improvement in the past year as far as being kind to strangers. Thanks in advance to everyone who gives suggestions....and HAPPY HOLIDAYS>>>:)