From Rob... I have a very loving and affectionate 3 yr old Jack.
For reasons unknown he will suddenly snap and bite, sometimes with a snarl before sometimes just snap. It does not need any provoking - he could be just laying there and just snap.
He knows what he is doing is wrong can see it after the fact in his motions. I am not sure what to do or what steps to take to cure the problem.
As much as I'd like to be able to help you out here, aggression issues can't be solved without working directly with the dog in question.
There are many reasons why a dog may show aggression - fear (from being traumatized in some way in the past), too much play fighting or rough housing as a puppy, to name a couple.
I recommend that you find a trainer in your area who has done a lot of work with aggressive dogs and who will have insight into your situation.
A balanced trainer (who trains using both reward and correction) will be able to do more to help you and your dog than an all-positive trainer in this case, as you cannot "ignore" aggression or solve it using treats.
Get professional help now, as it will only get worse with time, not better. This random-type aggression can be the most dangerous, as you never know when he may bite.
All this being said, the right trainer will be able to help you solve the problem with time.