Dear KAD,
Well if the episodes happen 3-4 times in a year, then we should go for an expert opinion. The good thing is that- since 3 years the duration or intensity has not increases, so we should not be scared due to this.
It can be due to -
1) Stress,
2) Very rare possibility of TIA.
3) Also a rare possibility of Migraine.
4) Slight evidence of Ambylopia.
SO i would recommend you to consult a good Neurologist for the opinion.
You have mentioned correctly that MRI can't be taken . But we can do MD-CT scan which gives fairly good idea about the things happening inside the head and brain.
Feel free to discuss further.
Regards
I also forgot to add, I have a small PFO which they recommended no medical intervention for.