Vision problem, eyes shut off.

[rdwood]

My daughter is 14 years old.  Generally very healthy.  She does suffer from periodic Migranes.  In the last 8 months or so she has been experiencing temporary visin lost.  She say's her eyes shut off for about 5 seconds, twice every hour.  It's like someone is controlling the light switch and flicks them off and back on.  Recently she started swimming for the school swim team and after about anhour of swimming she says they flicker on and off about every ten seconds and do this for and hour or more.  She has had every test under the sun (it seems).  They all come back normal.  

Thanks for any suggestions.