It worries me when you say your grazing is ALFALFA, CLOVER and grass.
Alfalfa and clover can be major triggers for all sorts of problems ranging from head flicking to nutting off at nothing. This is because both of these have an extremely high potassium load which causes major imbalances in potassium/sodium/clhoride ratios - all of which affects the nerves.
Personally I would not put a horse in a paddock with much clover or alfalfa but above all make sure that you are feeding SALT around 3 TBSP a day (yes tablespoons) EACH. Even if they have a salt lick, unless you live by the sea, they won't get enough out of a salt lick.
There's a really good article about this here ProvideIt Multi Vitamin & Minerals for Horses