Unusual aggressive behavior

Hello,

I have a fairly strange problem with my dog, who is a working border collie of about 8 years and male (entire). The basic problem is that he is aggressive to strangers, but it's unusual because what he tends to do is approach a stranger wagging his tale and very friendly but as soon as they stroke him or talk to him he will suddenly become very aggressive and snap his teeth at them, in some cases very close to their hands. He doesn't do this to everyone, some people he is fine with straight away but others he is not so good with. He is also fine with people provided that they completely ignore him. As of yet he hasn't ever actually bitten anyone but it can be a little too close for comfort. Does anyone have any idea how to tackle this problem and what might cause it? Any help is much appreciated. Thanks

I wouldn`t let him approach strangers if he is a bit iffy, personally. Why not call him away?

Has he had a health check at the vets lately? Might be an underlying physical problem that cannot be "seen" by you.

The behaviour really needs to be seen to see what is going on. There may be subtle differences in the tail language and body language when he approaches those he is fine with and those he isn't which can only be identified if observed.

Have you ever taken note of how the people he is fine with make contact as opposed to those he snaps at? Often dogs do not like being approached over their head, but usually learn to live with it. Could it be that the ones he is fine with go to pet him under his chin rather than over his head to get to his neck/back area?

What's he like when you, or those he knows well examine him around his head/ears/neck area? Is there a difference in how he reacts dependent on where you are examining/approaching him?

How long has the problem been going on? When it first started, did he have any ear infections or other medical problems? Does he have any now?

How old was he when you first noticed the problem and how did he react before that first incident? How old is he now?

What is his general behaviour like?

Difficult to say without seeing it but maybe he is giving out anxious, appeasement, deferential "flirt" behaviour (tail wagging approach) that is being misinterpreted as happy, relaxed and friendly?

His next response would then depend on how he judged them based on scent, voice, body language, timing etc.

People tend to think dogs who approach wish to interact and be petted. Often they just want to get close enough to check out if someone poses a threat or not, the last thing such dogs want is someone they are undecided about focusing on them with direct eye contact and reaching out.