German Shepherd personality changes

Question
My 1 1/2 neutered German Shepherd male recently bit a friend. The male has been increasingly aggressive, mostly at night. He's well trained, walks 3 miles with me almost every day.  He is about 91 lbs and is very tall.  He sits to his side which makes me think he might have sore legs, also he pulls himself up by his front legs - the vet has not said there's anything wrong with his legs.  I also have a non-spayed 7 month old female.  Here's my problem:  The male GSD shows strange aggression, unpredictable at times.  He's very smart and always follows my direction and the direction of my son.  However, on occasion, he becomes a "different" dog.  I would explain it as bi-polar if he were a person. He's fantastic most of the time, but about 20% of the time he growls and snips (in the evening).  His ears are back, he goes after the female and he doesn't want to be touched by anyone. At times, I feel like he's fighting his urge to attack, white eyes, tense body.  Is it possible he has a psychological disorder?  I don't understand why he's so wonderful most of the time and then changes...after two serious bites to a friend, I'm concerned about keeping him as a pet.  My vet says it might just be a genetic disorder.  Any thoughts?

Answer
It sounds like he has a real problem, and it likely will be worse in another year and a half when he is mature.  It is quite possible your vet is right.  There are many people breeding dogs that don't have a clue.  I get question after question from people with a dog with a behavior problem.  I tell them to neuter the dog.  They can't, they are going to breed it.

I am afraid fixing the problem is well beyond what I can help with.  If you found a good private trainer, you may be able to save him.  Try to find one that has some education and training.  There are too many ones out there that for lack of knowing how to correct the problem, will tell you it is simple, give him a 3 hour walk every day.  That would only tie up every evening from 7 to 10.  Really, there are people taking money for that advice.