
This sounds all very familiar to me. I've had Copper at training, behaviourists, the lot. He's the perfect dog. Responds (although I did have him muzzled at the training class), does the commands, got a special mention for effort.
The minute I get him home and try and walk him and continue the commands while walking the streets, its a no goer. He just won't do it. Barks at dogs, pulls, misbehaves, the lot.
I no longer know what to do with him except have my own routine of walking him at quiet times, avoiding dogs on sight and continuing to train and socialise him at home. I know that part of the problem is my handling of him which I'm working on but I've just had to accept the way he is and do my best.
That's all good and well with a small border terrier but a border collie is a big dog. Do try a behaviourist but be prepared for the most intense training work which you have to do. It is definitely NOT a quick fix. I'm also going to try "calms for dogs" which is a kind of alternative de stresser for dogs. Supposed to be good for aggression, but we'll see.
Good luck with the dog anyway, you can only do the best you can.
CG.