Are they silver or grey?
If grey, probably just floaters. I've got one permanently lodged in my eye jelly just off center (so if I try to focus on it, my eyes skitter down and to the left, very annoying).
If silver, certain things can cause "sparklies" to appear in your vision, such as if your blood pressure is low. These, for me, almost look like silver coloured fire works going off in my field of vision (the big, typical blooming ones). I don't see why they couldn't be streaks.
Third possibility is just a guess, but maybe some wierd form of hypoglycemia? Do you have a head ache at all before/after/during when this happens?
Finally, you could possibly have some sort of neurological misalignment or ailment. Err, that sounds bad, but I mean something along the lines of synesthesia (but, err, not synesthesia). A possibility may be a tumour, though unlikely. Have you had a CT scan since these streaks started appearing?
Of course, there could be some other bizarre reason, but I would think it's probably physiological from what you describe (and I get from your post you seem to think it is physiological as well, correct me if I'm wrong).
Some more description would help, if it is at all possible.