E. Springer bad behavior Discussions

We have a 2 year old Springer named Jasper. We have had him go through a training course starting when he was around 6 months old. He is reasonably obedient in most cases and he is very loveable dog. We do have some issues that have happened on about 4 occasions over the course of the last year. On these occasions, he has viciously snarled and snipped at a person who was trying to pet him. This has happened to our grandaughter (6 yrold), a visitors young son, a 10 year old boy and a grown man. In the case of the children, Jasper was fine around them until they started playing as kids do and they decided to pet him when he snipped at them while snarling at them. The has not totally bitten any of them that broke skin or anything, but we are greatly concerned. The man he snarled out was a worker, who after the incident was able to play with Jasper.
We are retired and have not been able to get Jasper out and socialize with him in public. He barks at anyone that comes, but in most cases, settles down quickly and then is fine with them around. A vet informed us when we first got him that he thought he had some vision problems. He later said that it was not much of an issue, but I am wondering if he might still have some vision issues and sees there reaching to pet him as a threat to maybe some bluriness maybe. Or if his lack of socialization may be the issue. He is such a great dog almost all of the time except on these rare occassions.
I am wanting to know if anyone else has had similar issues or any advice as to what may help.