I started getting these when I was 51. I was in hospital at the time,just been put on a anti-biotic drip, so a few doctors and a pharmacist looked at me. No one was really sure what triggered it, but I've had them every few months ever since. I've tried to pin it down to some common factor, but there doesn't seem to be one. Stress or my age is all I can think of!
Make sure you have seen an ophthlamologist/eye MD for this and had a full dilated eye exam. If still concerned, you could then see a neuro-ophthalmologist for assessment and recommendations.
HV