Hi Cathie,
I don't have any personal experience with that but have read a book that you will probably find extremely useful, probably doesn't give you a solution but at least some information.
Fed up by Sue Dengate, here's her website:
http://www.fedupwithfoodadditives.info/. The author is known for her efforts to lobby the Australian food industry to stop adding preservative 280 to breads because she was able to show a link between it and childrens IQ, concentration span & moods. So now, none of of breads in Australia contain this preservative

.
Chocolate is a VERY common food to cause intolerances, which is different to an allergy, symptoms show up over time, when a critcal exposure limit is reached in the body. Chocolate is high in amines, which commonly causes migranes, but there are other effects also. The book explains different foods that cause slow reactions - intolerances and how food can effect kids behaviour.
Foods that cause intolerances can be grouped into:
- amines
- preservatives
- Artifical flavours
- glutamines
- salicylates
- artifical colours
- dairy
-suphites
- benzoates
- gluten
The authors recommendation is to go on a diet for a few weeks low in all of the above, then do a series of food challenges with foods from each category to see what you are sensitive to. What I get from the book is that there is no use in eliminating one food only, e.g. chocolate only, if you are still eating other amine rich or other foods known to cause intolerances. The results of your test will be confounded by the other foods (variables) that could cause intolerances. It sounds like alot of work but the rewards sounds amazing. There are also recipe suggestions and shopping lists in the book.
HTH
Eloise