I've contacted a behaviourist and waiting to hear back.
In the mean time...I'm at a loss. My super friendly male dog started (a couple of months ago perhaps) growling at my partner on an evening when we were sat in the living room. Then one night he just flipped and flew at him. To stop it from happening again he is now in his crate in the living room or on a lead as I just don't trust him.
We had friends over Saturday night and he was very wary of them and took himself to his crate (I gave lots of treats) but he kept coming out and would be happy then just flip.
The only common theme is that it's always on an evening. During the day he's fine but then again we're not chilling on the sofa as a family.
Walk wise, he gets a lot of exercise and mental stimulation. He's fine with most dogs but again, can randomly flip and he breaks/growls/lunges. I don't think there's any common denominator with other dogs and I can't work out wether it's him feeling frustrated or threatened.