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.