My dog is occasionally vicious

Question
We have had our cocker spaniel for 6 years now, and every now and again at completely random times; he will go to bite us if we go to stroke him. He began this when he was about 4 years old, doing it about 4 times a year maybe at random occasions.
My parents want to get rid of him, but i don't think we should be giving up just yet.
What do you advise?

Answer
I need to know exactly what he's doing when you pet him and he snaps.

Is it when he's eating?  Sleeping?  On furniture?  In his bed?

What do you do when he does this?  Does he do this with everyone or specific people?

In order to change this behavior we need to figure out what's triggering it.
Delores