All sorts of things can make a dog anxiouse, they pick up on body language as well as smells and noise, its suprising how far and how sensitive they can smell and hear. He could be hearing of feeling something that only he can hear.....are you near one of those public buildings with anti teenager alarms that can only be heard by young people?
First point of call is is he unwell? dose he also have the runs, high temp vomit etc when he has one of these anxiety attacks, get that checked out maybe first, then is his food right for him, some dogs get very hyper on some food brands, its quite common for a hyper dog to calm right down when the food is changed, although the shaking etc sounds a bit more than just food, poor thing and poor you, it must be a worry.
I would make a note of times he is being odd and see if there is anything going on, it could even be something like an electric relay station buzzing and winding him up, then exhusting him!!!
Keeping notes could indicate that there is something going on....I never forget the story about a farmer whos dog howeld when the phone rang, turned out the phone cable connected to the post he was tied to!!
I hope its something easy,

Oh just had another thought, could he be eating something toxic, Labs are so good/bad at finding stuff they should not eat!