I've been having these for years.
First time was while watching the 2006 World Cup.
The ball kept disappearing as I tried to follow it across the screen. Very worrying.
From there I had them, maybe, twice a year.
Last year (2012) they came every month.
This year none.
They always last about 20 or so minutes.
I'm still not sure what brings them on as I've had them when perfectly relaxed.
Could be a blood pressure thing.