My husband and I have two dogs, both were rescues. Sophie (a spaniel mix) that we got as a puppy and is now 6 yrs (yes, spoiled), and now Max we got a year ago. They thought he was a Jack Rat Terrier mix and maybe a year and a half. I think he is a Basenji and we've had him now a year. Max and Sophie get on so well and there's no problems there. Sophi will put him in his place if he gets too much and they play well. It was about three months when Max started displaying dark moments and agressive hostility towards my husband. Vet said he was an Alpha dog and to work with a behaviorist. It was almost a "Me or the Dog" for a whlie and I would have hated to have got rid of my husband after 28 years (just kidding) but we have survived until now with adjustments like a crate for Max at night. I will not trust Max with kids but it's just us and we can learn to live te a "special needs" dog. At this point I guess I should I post details and questions about this at the Behavior Forum? I just wanted to intro ourselves as found this site today and there's some good advice here and we need it. Thanks.
The first place to start is to get your Max's Thyroid tested. The basenji thyroid is quite different than other dogs and can cause aggression, and personality changes.
Here is a place to start reading and to get more info.
Welcome to the forum
Thank you and glad to be here Thunderbird! I see you are from the UK. I'm a transplant to the US over 30 years ago (originally from Essex). My sister said she got snow today. This is such a great forum and I've gotten a wealth of info and ideas and everyone is so encouraging. I'm not going to give up on our Max! He has such great spirit. Nice to meet you.
Try to find somewhere that offers the IDEXX snap 4DX test. It tests for heartworm as well as lyme, erlichia and anaplasmosis. The last 3 are all rickettsial diseases that can cause behavioral changes as well.
If medical causes are ruled out, there is a great group on yahoo groups called agbeh. It is the Aggressive behaviors in dogs group.
It is very strict due to the sensitivity of dealing with aggressive dogs, but the files and links on the group are great, and reading through the archives is invaluable.
-full disclosure: I am not the owner of the group, but I am a moderator for the group.