I have written several pages on this subject over the last few years. Frankly, some of it is a bit frightening, but the severe episodes are usually associated with migraine that causes large areas of blindness, followed by severe headache and sickness. I doubt that this applies to you.
I talked about late night conversations with aircrew who, after many drinks, would confide in their colleagues about quite personal matters. It was obvious that some of them had suffered migraine. It was obviously festering in their background thoughts all the time.
Zig-zags seem to be a type of migraine that often starts in later life. It is also a type that is often not very severe, however, as you say, the visual areas that were affected remain ‘vague' and this alone means that the visual processing in those areas is not reliable.
The problem is, that flying is utterly reliant on eyesight. Even a small disturbance could be critical on a dark and stormy night. Someone recently wrote on this forum about always fearing the onset...always checking if a blind spot--caused by a reflection in the intense light at altitude–was the first migraine blind-spot. It sounded like an ongoing mental torture.
Do a search for my postings, if no luck let us know and I will look for my notes.
I'm now not so convinced that day to day stress in not causal. Deep underlying worry was, I felt, the main cause, but now I'm not sure. I'm still fairly sure that chocolate / cheese / turkey etc can trigger an episode, despite there being a need for two other factors in the brain chemistry to allow Tryptophan
http://en.wikipedia.org/wiki/Tryptophan to cross the blood/brain barrier. It all gets rather complicated at this point, because the main symptoms of migraine are caused at the dilation stage, not the initial constriction.
It is my belief, that if one could stop this natural constriction, the problems would go away.
Your life at the time of the onset is indicative of the cause. Deep relaxation may help. In some cases there has been a suggestion that simple imagery of the brains blood vessels, opening and allowing more flow, can stop the constriction/clamping in the first phase.