Having a problem with my kitty

For the past few months, Pepper, my 1 year old spayed female kitty has become very agressive towards the other cats. She will attack without warning and most often attacks the other female, Isabelle.....but she does attack the male on occasion. Isabelle will be sleeping and Pepper will be walking past and suddenly fly at her and attack her. These attacks happen several times a day and are always without warning. Isabelle luckily has not received any scratches or bites from Pepper......which is a really good thing because Pepper has tested positive 3 times for FeLV but has shown no symptoms, and according to the vet she must have an "inactive" form of it. My two other cats have tested negative several times. All have been vaccinated for it, so we aren't sure how Pepper ended up with it.
Anyway, the vet cannot give me an answer as to the agression. Is it possible that the FeLV is causing this sudden agression? The vet said maybe she's just not feeling well, but she eats fine, doesn't have a temperature, runs around the house and plays all the time....and is just a very active kitty. She is extremely affectionate towards people, she just has it out for the other cats. She used to LOVE the other cats, especially kittens. She and Woody (the male) grew up together since they were 9 & 11 weeks old. When Isabelle joined the household at 4 months old, Pepper would sit and snuggle with her and play with her and would treat her like her own baby.
Anyway, I was hoping that someone had some suggestions. Also, if anyone has any good information on FeLV, I would appreciate it. Most of the information that I find just talks about the symptoms and testing, but that's about it.

This group might be able to offer you-

http://www.felineleukemia.org/mail.html

I would also look into a Feliway Comfort Zone Room Diffuser and if the attacks continue then run your kitty to the vet for a quick check up and bloodwork. Sudden acts of aggression can be a response to pain-