One form of aggression in this species can begin as the males become mature. Their personalities sometimes undergo a significant change, and biting can be one of the results. If biting persists consult a veterinarian who treats rats. The vet may recommend that you have your rat neutered.
It is possible the rat is dealing with pain from mouth burns from chewing the electrical cord and pain can make any animal grumpy. For that too a vet visit might be the best choice.
Here is a site with recommendations on vets who will do rat care
http://www.ratfanclub.org/vetref.html
Hope this helps you!
Pet Health Care, Rescue,Train,Breed
30+yrs pet vet care & nursing, rescue, behavior & training, responsible breeding, small animal care
I'd suggest calling local vets and asking who in the area does small pet care like this.
Here are some CA vets to check
http://www.rmca.org/barmca/veterinarians.html
I saw one in San Francisco who may be close enough or able to tell you someone closer.
Thanks for your above accept!