Melholly - you obviously have experience with dogs and sure you will get there in the end, as he seems to be basically a good lad. Was he kept in his cage a lot where he was before? ...... not just at night but in the day? Could he feel that the cage is the "house" if you like and so once let out, he is free to empty himself? Just a thought.
Doganjo - I think it may depend on your definition of"nervy". Maybe "over-sensitive" is a better description of SOME working labs. Top lines are bred very specifically for a certain job and it is believed that such breeding for a biddable nature has led to some individuals being if you like almost too biddable ..... "sensitive", "nervy", ..... too anxious to please. You are right they are not as "high energy" as some breeds but it doesn't necessarily follow that they are not sensitive souls. My Flattie, true to type for his breed, is more high energy than a working lab but equally not as sensitive. If you watched the working gundog display at Crufts, it was said that you could train/handle 3 working labs, compared to 2 Goldies, compared to 1 Flattie. Labs., in general, are more "push button". They are very biddable and hence easier to train. They are easier to train because they are more sensitive.
My father has 2 working labs. at the moment. Top lines and super workers. Hard on game. If you ask him about their temperaments as pets/house dogs, he will tell you they are not the best. How you describe them would depend on your interpretation of their behaviour - "nervy" maybe. He has had others with very sound temperaments, all treated the same way. My neighbour has a working lab. A beautiful lad, with a super temperament but again a "sensitive" dog.
Just my opinion. However, if one knows top handlers of working labs, then you will find it discussed quite openly and even assumptions made as to which lines help to pass on these "sensitivities".
I will agree to disagree, though.