Nasty little blighters, these things.
I've been getting them on and off for ten years. No apparent pattern, although I go through a phase of having them up every couple of days/daily for a week or two, then no more for three/six months or more.
They are quite debilitating when they come along; although cause no direct pain. I certainly wouldnn't be able to drive when one is in full flow. Start off as a pinprick of blurring and then grow slowly with the sawtooth pertimer as Rivets describes. Eventually you can see clearly through the middle of it, but can see nothing at the point where the sawtooth marks are. Tend to last up to 30 mins, and eventually just disappear outside of peripheral vision.
I'm not convinces that they are in the eye; they feel more likely to be somewhere behind where your brain processes info - I usually have a single blur that grows over both eyes vision and remains in the same location if I close each eye in turn. Does normally start in my dominant eye though.
Doc has in the past said that causal effects can be fatigue, stress factors and dehydration/caffeine abuse...I plead guilty to all the above! Whilst I'm not qualified to offer ANY medical advise, I have in the past been able to kill them off as soon as I feel the first symptom with 3 x paracetamol and 2 pt's water double-quick (I'm not normally a big fan of pill-popping)
Could even be something pyschosematic in them too...saw this post yesterday and meant to reply; then hey-presto I find myself writing this through the middle of an episode!! Months since my last one - all very strange.
Also as Rivets mentioned, it could be deep-seated worry (sub-conscoius) - I lost my job last week, and whilst I'm quite chilled and optimistic; who know what lurks beneath....