Hi James
Hmm this must be very frustrating because it is so unpredictable but i am glad you have asked for help.
I think some alpha training as well as general obedience will help to a large extent in this situation.
I am not sure what the triggers are. Are there common ages? adults vs kids? or hair colour? Think of anything in common and be wary in these situations.
The first advice i would give you is to ensure you and your family members have read and understand the techniques in the bonus book "Secrets to becoming the Alpha Dog". These are great techniques for maintaining or establishing your position at the head of the household. No matter what the problem is all dogs need to know where the stand in the house for both yours and their peace and comfort.
Here are some ways to reinforce your position-
1) If you come across your dogs while they are sleeping or lying on the floor then you can reinforce your position as alpha dog by making him move so that you can pass by.
2) Make sure that you always go through doorways first. A good method to reinforce your position as alpha dog is to walk your dog around the house on the leash, making your dog wait while you walk through doorways first.
3) At mealtimes make sure that your dogs eat after all of the humans have.
4) Do not feed your dogs tidbits or let it pester you at the table. Save the morsels and tidbits for training sessions instead.
5) Do not greet your dogs straightaway when you arrive home. Make it wait
until you are ready and then call it to you.
6) Whenever your dogs want attention or anything wait till they are sitting and being well behaved.
7) When you give a command make sure that you are in a position to enforce the action that you require from your dog, especially in the initial stages of Alpha Dog training. Also, use the Alarm-No-Command technique as described in the Alpha Dog bonus book to reprimand your dog if it does not obey your command.
The other major thing that will help. It is good that he can sit but we want him to sit, stay and come in all situations. Start training him for10 minutes twice a day on these basic commands. Once he has started to respond then try other situations where he may be less obedient eg on walks and around other people.
Until you are sure what the trigger is it may be best to have him on a muzzle.
Once you have more of an idea and have tried alpha and obedience let me know and i will see if i can help any more.
Good luck
Kind regards
Todd Field