First of all, skip the ADD/ADHD "diagnosis"...go to that as a very last resort. It is wayyyy misdiagnosed in this country for kids who are outside of the realm of "normal" (ie, "perfect angels".) The difference in the number of kids with ADD/ADHD who actually have it, and those that were rushed into a quick diagnosis is staggering. Parents who rush out to the doctor with their kid pre-labelled "ADD" are looking for a quick fix, and in turn, have no problem telling their actually normal children that they have a metal disorder. So much for little Susie's self esteem, eh? (BTW, I am not picking on you, and i apologize for coming across harshly...but I do have ADD even as an adult and it floors me how many people are so quick to jump to that conclusion. A real diagnosis of ADD is hard to come by...it's actually a final answer when al other potential causes are ruled out...and the almost purely American epidemic of ADD/ADHD diagnosis comes from lazy doctors and parents desperate for answers.)
ADD/ADHD has nothing to do with empathy. You said she will be good for a little while, and then the old behavior is back. So, obviously, it *is* controllable, even for a short while. Does she show signs of sadness for hurt animals, or when mommy is upset? does she play with dolls, hold them, cuddle them, dress them, brush their hair, etc? Does she have a teddy bear she holds at night? Does she give hugs and kisses at night before bed? Any yes answers to these show she DOES have empathy and does feel compassion.
She is probably just a little slow on her social skills, which is still within the realm of normal and does not call for counseling or therapy. More than likely, if anything, she is a part of a vicious cycle where, perhaps she had a bad day and lashed out or said something mean, kids started calling her names, which made her feel worse and lash out. Perhaps she tried being nice once sometime ago and who she was nice to was mean to her and she internalized it. If she has heard you and your husband or anyone else says she is a rude, mean child, she is simply living up to a definition and label that has been put on her. Or, she is getting attention from her misbehavior...she gets talks and lectures and even if it is negative, people are still talking about her.
What to do? Keep on keeping on. Keep showing her how she needs to behave with others, and when she crosses a line, let there be consequences...either one you set (eg, time-out) or natural ones (if you are ugly in behavior to me, we don't play anymore.) And focus on catching her being good. If she so much as says thank you, reward her! When she is ugly, (at home) ignore it....have your communication with her follow the 80/20 rule...80% + positive, 20%- negative...It will seem silly at first and counterintuitive to ignore her insults and rudeness (it may even get worse at first), but she should grasp the concept of "If I am ugly, I get nothing...If I am sweet/friendly/positive, I get rewarded" soon enough.