You could be having a food reaction with vitamins. Perhaps you could keep track of what you eat and when you have those episodes you might be able to ascertain what it is causing the reaction.

You might want to change brands of your vitamins? Do they have anything in them that is "special"? Like lutein? The inconsistency of it leads me to believe it must be related to some food you eat at times though. Were you eating any kind of fish or shrimp at the times of your reactions?

Good Luck!